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Frequently Asked Questions

Mikuni Carbs

Mikunis for Triumphs

Q What size carbs for my GT6

5/13/2014 1:41 AM, Mark Potter wrote: Mikuni’s for my 72 Triumph GT6
Currently it’s a stock motor with about 15k on it. I’ll be adding a performance 6 into 2 header, and pacesetter exhaust now. In the next year or two I’ll likely add a somewhat aggressive road cam, and possibly increase compression a bit.

So.. fairly mild upgrades. Would the 45 pair be the size I’m looking for? 

Mark,

For 2 L engines, especially stock, I usually recommend the 42 mm carbs.  For extensively modified engines, competition applications, larger displacements like the TR6 2.5  I recommend 45 mm carbs.  There is only about a 10% difference in their flow rate so the performance differences are not going to be large.  If the car came with 1.5 inch carbs, then the 42 mm mikunis ( 1.65 inch ) are going to be slightly easier to adapt than the 45s at 1.75 inch.  Fuel economy and low end torque could be slightly better with the 42s, while max power will usually be better with the 45s - but again these are differences in the 10% range.  Cost is the same.

Regards,

John

Vintage Performance Developments

 

Q   TR4 Race Tuning

Tuesday, May 25, 2010 10:55 AM
Subject: HSR's on a tr-4
 

John,

I think I finally have my jetting sorted out ( for this altitude ). I have been having a bit of a time getting

my exhaust gas temps down to 1200 & under. To do that I have had to go to the richest needle jets

and jet needles I can find.

I have 195 mains in it now which is giving me about 12:1 AFR.

The problem I have is in August I’m going to Heartland Park in Topeka for a week.

The elevation is 945’ which is a 4175’ change. If I’m using the richest jetting available

for 5120’ I don’t know what I will do when I drop 4100’ in elevation.

The main jets I can deal with even if I had to drill custom jets but part throttle

( ¼ - ¾ ) is where my EGT’s go above 1250. I have had to turn in my air screws to offset the high

EGT’s.

I just ordered a set of Yost power tubes in hopes they will fatten things up a bit.

My car is right at 2.2 liters, what are you doing jetting wise for cars with similar displacement

on two carbs? On your website you mention re-profiled jet needles, can you tell me a bit about those ?

Any help would be appreciated. I’d hate to get to Topeka and find out I can’t jet the car for the altitude.

 

Jim Gray

A

JIm,
 
Since this is a racecar, I don't understand your concern with part throttle exhaust gas temps.
 
There is no indication here that you have had it on a dyno.
I would not go by egt and afr alone.
You might find that hotter leaner gives better power.
Also look at the effect of the ignition timing on this.
too much or too little advance can cause high egt.
What is your timing at 4,000 + rpm?
 
There are an infinite number of needle profiles, though the taper, which is what is usually changed, mainly affects midrange acceleration when the throttle is opening, and I am not sure that is your issue. 
For a richer midrange at part throttle or as the throttle is opening, cut a more acute taper into the needle so that the constant diameter tip is longer than on the stock needle.
If its abrupt acceleration where it goes lean, then enlarge the acceleration pump jet.
If its still not getting enough of a accl. pump shot, then go to a larger accl. pump. piston.
 
But I would really like to see a dyno chart including AFR ratio throughout the run as a single egt and AFR doesn't tell me anything usefull as they could change radically based on load, throttle opening, rpm, ignition timing, etc.
 
My race engine with light or no load ( 45 DCOE webers ) , part throttle will run 1600 - 1700 on the egts if I just let it sit there at a with a steady throttle without a load. The header will start to glow and eventually melt.  Open the throttle and let in some fuel and the temps immediately go down.
Richen up the idle and part throttle circuit and it will bog when the throttle is opened at below 5,000 rpm.
Its a racecar, so its never run at part throttle on the track.
 
 
Regards,
John
Vintag Performance Devolopments

 

 

Q

Sunday, February 21, 2010 10:05 AM

Subject: Triple Mikuni Throttle Linkage

John:
 
A member of the TR6 club 6-Pack shared your web site on our forum focusing on the Mikuni Carburetor.  I acquired what I believe to be a Prirace triple Mikuni HSR45 conversion from a member of 6-Pack and replaced my Triple Zenith Strombergs.  It did not come with the accelerator component which I don't think I would have used.
 
I kept my stock accelerator petal shaft in place and connected the 3-1 cable to the accelerator shaft lever.  See attached pictures. 
 
This cable sticks open a little when returning to idle and the petal feel under foot is not very good when cruising.  Maybe it is the bends in the cable.
 
I see on your site that you have fabricated a throttle shaft set up to replace the carb cable.   I would like to return to a shaft lever pull to the carbs throttle shaft without the 3-1 cable.  Or reposition the cables so to get a better throttle feel.
 
Do you have a throttle shaft conversion for the triple Mikunis replacing the carb's cable pulls? 
 
I can see that I will have to tilt the carbs to line up with one another's throttle shaft instead of having them plumb as I do now.
 
Your thoughts recommendations appreciated.
 
Thanks,
Al Gary
 
 A
 
Al,
 
I had not seen the Prirace kit until getting your email.
My first reaction is that I can imagine real problems with keeping the carbs sycronized when they are only attached by cables.
In our kits the carbs have all been modified with extended throttle shafts which allows the shafts to be connected and can be used in groups of 2,3,4 or six carbs - all with a direct connection to each other. This adds considerably to our cost for the carbs, yet we still sell the kits for hundreds of dollars less.
 
The carbs were designed to have a two cable throttle setup - one to open it, one to pull it closed.  Its not necessary to use two cables in an automotive setup, but it is usually necessary to attach at least one external throttle return spring. With a three cable setup it might be necessary to have an external spring for each carb since the cables work well when pulling to open the throttle, but not very well when trying to push to close it.  Each carb has provision for a throttle return cable, and this can be used as a neat way to attach a throttle return spring. 
 
Another issue with the pure cable setup is that there is no provision for adjusting the mechanical advantage ratio between the throttle pedal and the carb throttle shaft. In other words the amount of throttle pedal travel vs. carb thorttle slide travel.  When a throttle shaft is used,  with the cable attached to a lever arm, this can be adjusted by changing the length of the lever arm or where on the arm the cable attaches. 
 
As to your specific issues.
First you might want to try installing an external return spring on each carb.
The second cure is what you suggested - installing extended throttle shafts in each carb. With that done you would have the option of attaching a single throttle cable either to a bracket on only one of the carbs, or to a lever arm on one of the intermediate shafts that attach the two carbs.  The extended throttle shafts are $50 each. The couplers are $25. The lever arm is $5.50.  Shafts between the carbs are $10.50 each.
 
Please let me know if you have any other questions and pass on my comments to other members of your forum.
 
Regards,
John
Vintage Performance Developments
 
Note:  Our kits for the TR6 use carbs with custom made extended throttle shafts for a solid connection between the carbs and use the stock throttle linkage with a minor modification.  Customers prefer this over using a cabel type of throttle linkage.
 
Mikunis for Datsun
 
Q
 
Saturday, February 20, 2010 6:30 PM
Subject: HSR for Datsun 510
 

G’day,

I stumbled across your site while investigating carbs for my current project car. It’s a Datsun 510 and Im building up a stroked and bored L series engine that will be about 2.3litres and good for around 8500rpm. Just wondering if you think 2 HSR’s would be enough on a twin SU manifold (similar setup to what you have photos of on your site) or would I need to go to the quad setup on a twin weber manifold. I think the standard SU’s are 38mm (1.5in) so would the manifold need to be opened up for the HSR to suit whichever size you used?

 Also noticed on the photos of the 510 setup that there are no heatshields to protect the carb from radiated heat. Are the HSR’s capable of handling more heat than the SU’s or would heatshields be recommended.

 And of course what sor of costs for shipping a kit to Australia.

 Would be interested in your thoughts.

 Thanks,

Stuart

A

Stuart,
 
The best answer I can give you on whether two carbs would be enough is that they would be on the right manifold.  Its probably the manifold that is going to be the limiting factor.  In testing on a 2.5 liter Volvo B20 it was the SU intake manifold that became a limiting factor over 6500 rpm and limited HP to approx. 195 at the crank. Whether the Datson intake would be any better I can't tell you.
The Volvo intake was unmodified, and doing some work on it might have made a difference, but at the time we doing some quick comparisons without any time to do much tuning or mods during the process.  As a point of reference, we also tried three different manifolds with 45 DCOE carbs.  Two limited HP to the 200 - 205 range. The third, a longer manifold, allowed us to get over 210 without any other changes. 
 
And, yes, the intake side of  the manifold would need to be opened up to match the size of the Mikuni.
 
I would also use a heat shield. A customer did the setup in the photos.
 
Assuming that you have a stock intake manifold, you might want to try it with 2 Mikunis.  If it doesn't work you can use the two as half of a 4 carb setup.
The Quad Kit listed on our website is essentially just two of the dual carb kits.
The only real difference is the need to use a different type of air filter when using the carbs in close coupled pairs.
 
Shipping to Australia by priority mail for the dual kit, without air filters would be $68.55 for the dual kit, $109.45 for the Quad kit, both without air filters.
 
Right now the 45mm carbs are available, but due to the current demand the 42 mm are back ordered but should be available again by the end of the month.  Once they come in they have to be modified for automotive use, so we have to have them for a few days before they are ready to ship.
 
Please let me know if you have any other questions.
 
Regards,
John
Vintage Performance Developments

 

 Carb Choices

Q -

Thursday, September 10, 2009 3:00 PM
Subject: volvo carb
 

>I recently purchased a 1969 volvo p1800 with original dual stromberg
> carbs.  I have been told the best/most fun way to drive this vehicle is
> with a Weber dcoe side draft set up. Can you advise me on what is
> needed for this (carb, intake manifold.linkage etc.) and if you carry
> these items.  Thanks
>
> Tim

 

A -

Tim,
We can supply sidedraft weber carbs and parts, but I don't recommend them for a street engine, especially if it is unmodified.
Although I have used Weber DCOE carbs for maybe 30 years, and still use them on our vintage race cars, I developed the dual mikuni kit several years ago as it was a much better way to go in terms of a replacement carb kit for street cars - power and thottle response at least as good as the Webers, easier to tune, better fuel mileage, more reliable, parts are cheaper, etc. They are simply just better carbs that take advantage of 40 years of technological advancement since the Webers were designed.
 
Regards,
John
Vintage Performance Developments
 

 

9/3/09

Q -

i was searching the internet for carburetors and found your site. Great site with great stuff, only i do not own a Volvo, but a Triumph TR-4. I saw the photo you have on your site of Jim Gray's TR4 with the Mikuni set up. I have thought about this set up as opposed to using 2 45dcoe's, but as yet, I'm still undecided. I have never actually talked with anyone who is running the Mikuni's on a TR4.
 
I've almost completed my engine build and the shop that did my porting sized my cylinder head intakes for the Weber's using a Canon manifold from Redline. Can this manifold be fitted to the Mikuni's also? Can the set up you show with 4 Mikuni's also be used on the TR4?
 
My car will be used for both street and some vintage type track events. I don't plan on racing the car exclusively. The engine build has been a long and thougtful process (expensive as well) and I've tried to incorporate every performance modification I can to help the old Triumph engine reach it's peak HP and torque. Without getting into specifics, I'm hoping the engine will dyno close to 180 bhp.
 
Thanks-
 
Tommy
 
A -
 
Thomas,
 
It really does not matter much whether the Mikunis are on a Volvo, TR4, MGB, etc.
They are carbs with significant technological advancements over SUs, Zenith Strombergs, or even Webers. Several years ago I spent most of a year testing various different carbs, including some configurations and modifications most people have never heard of.  The winner by far was the Mikuni HSR with the best combination of power, throttle response and fuel economy. 
 
I use Weber DCOE carbs all of the time and have used them for at least 30 years.
Just got back from doing a dyno tune with them on a Volvo vintage racer.
We use them because they are traditional on vintage racecars and some organisations do not allow a more modern carb. Other vintage organisations allow any carb, but require the original manifold, and so the Mikunis are allowed.
 
The Mikunis will do things the Webers can't, including have a much higher flow capacity when comparing equal sizes. Because of their ease of tuning and better fuel mileage they are an easy choice over the sidedraft webers for a street car.
The only disadvantage of the dual mikuni setup vs a pair of 45 DCOE webers is that the original SU intake manifold with be a primary restriction at some point, around 190 HP in our Volvo testing.  The solution is to use the same intake manifold as the 45 DCOE carbs and use 4 mikunis.  This setup will flow better and have other significant advantages over the Webers.
 
So to sum up, I would recommend the dual Mikuni kit, and that should be sufficient for your 180 HP goal. If you have significantly more power than that go with the quad Mikuni setup.
 
Regards,
John
Vintage Performance Developments
 

Mikuni Tuning

Q

Sent: Friday, February 19, 2010 10:46 AM
Subject: Re: Mikuni on Datsun 510(
 

When I first drove it they (plugs ) were white idling around the block but I had way to much timing(Vac. Adv was sticking), it would ping whenever i tried to go past 1/4 throttle.  So I disconnected the vacuum adv. and set the timing back to 16 w/o any advance hooked up. Did a short test drive gave it good blast in third, cut the throttle, clutched and shut down and rolled into my drive way and check plugs. slight coloring.  Engine was pretty crisp no bogs or lags.

A

The mikunis should be tuned based on throttle opening.
Divide it into 1/4s and tune each segment.
Drive for at least 1/2 mile with the throttle at a steady opening, hit the clutch, turn the motor off, and coast to a stop. Check the plugs ( one is usually enough if you know they give similar readings ), make adjustments and do it again. Start with low throttle openings and work up to WOT.  WOT is best done on a hill to control speed.
 
John
VPD


 

8/6/09
Q - I wanted to get your advise on what direction to go with tuning my mikuni carbs.  I have been playing with the carbs for a few weeks now and have been slowly improving the performance.  I am still having some issues that im not sure how to fix.  First I have the accelerator pump adjusted to open immediately as i touch on the gas, my idel is running at 13.5 to 1 mix, but it still stutters for a moment starting out. I see the o2 sensor jump lean for a moment then goes back to 13.5.  It goes great untill I try to floor it and the shoots lean 20 to 1 mix and loses all power.  Im running a 175 main jet and im assuming I need a larger one for the full throttle.  But im not sure how to fix the starting issue.  What would you suggest.
 
A -
Rob,
I can't remember the details of everyone's engines, but here are some thoughts.
 
Drivers often open the throttle to 3/4 or even all the way for routine acceleration from a stop. This brings the carb immediately into the area where the mains are controling the air/fuel ratio. 175 is on the lean side for any of our setups. I would jump to 185 or even 190 on the mains and see what that does.
Its often better to take big jumps and then back down as needed.
 
Conversely, drivers think the mains will be controling when driving at a steady 70 - 80 mph. In reality the throttle is usually only partially open under cruise conditions, with the idle circuit providing most of the fuel.
 
13.5 air/fuel is lean for any accelertation. Best fuel ratio for acceleration is usually in the 11.5 to 12 range.
 
Try more ignition advance.
 
You are probably opening the throttle too quickly.  Try opening it more slowly.
If that works, you might want to modify the linkage so that it takes more throttle pedal movement to get the same amount of movement at the carb.
 
Also try not opening the throttle all the way.  Don't just assume that you will get the best acceleration with the throttle all the way open, especially at lower rpm. At wide open throttle the carbs may be able to provide more air then your engine can use. If that is the case, you might want to setup the linkage so the throttle slide does not open all the way.
 

MGB Carb Upgrade

6/7/09

Q - Installed your dual Mikuni carb kit on my 79 MGB in place of the single ZS carb. I am having trouble adjusting it. It needs a lot choke to start and runs on when I shut it off.

A - (As the car was not far away I took a trip to see what the problem was.)  The pilot air screws could not be adjusted rich enough to give the car a decent idle so I suspected a vacuum leak.  It turned out that the breather hose from the block was hooked into the intake manifold, causing what was essentially a vacuum leak.  Blocking off the hose was a temporary fix allowing the carb pilot air screws to be turned back to the factory adjustment of 2 turns out. A permanent fix will be to rout both the block and valve cover breather hoses to the air filters.

 

5/31/09

Q -  What are the best carbs or carb to use on my MBG?

A -  I usually do the answers, but where there are good answers from others I will post them here. See the answer given below on an MGB forum.       -      John


True the Zenith carbs can be tuned to give reasonable performance, but asused one at a time on rubber-bumper Bs and other contemporary B.L. cars they tend to be underwhelming in terms of performance. The single ZS on my ' 75 Midget was a public menace. The car wouldn't pull the skin off a rice pudding. When I replaced it with a pair of 1 1/2" SUs on a manifold from an earlier Spitfire the little engine just came to life.

Seems to me the stock carb on a rubber-bumper B gave a manly 65 or 66 bhp.,hardly what one would expect from a car with sporting pretentions.

That being said, I'm not a great admirer of SUs either, having spent many a happy hour replacing worn throttle shafts, bushing oval holes in carb bodies, even converting the things to ball-bearing shafts. The mobile-jet chokes are primitive to say the least. New SUs are expensive ( at least inCanada), and quality of the examples I've seen in recent years has not been impressive either, much like new Amals for classic bikes.

As mentioned in a prev. post, when I put the fresh engine in my ' 70 B-GTat 100,000 miles I chucked the SUs away and replaced them with a pair of HSR42 Mikuni flat-slides. These are simple, reliable, very well made unitsusually used on bikes ( particularly hotrod Harleys). They cost me less than a new pair of SUs, fit on the stock intake manifold quite neatly, and outperform the SUs by a wide margin.

Stock carbs for the pre-rubber bumper Bs were 1 1/2", and the special tuning manual called for 1 3/4" SUs for fast road work with hotter cams,increased comp. etc. etc. At 42 mm the Mikunis are nicely inbetween those two in choke size, and I suspect they flow better than standard SUs of either size. They are much more compact than the stock carbs, are mounted on rubber spiggots so are much simpler to fit or remove, and they can use awide array of K&N pancake or conical filters. Of course they are meant forgravity feed in bike applications so you need to put a fuel pressure regulator ( cheap) in the line in order to step down the pressure from the fuel pump. Due to the slight inclination of the stock manifold I had to adjust the float level slightly lower than stock, but once that was sorted it was a 'set and forget' proposition.

One serious benefit is that you don't require the aid of Merlin to set orjet the things. They respond to jetting changes and adjustments exactly as you would expect, which cannot be said for the old SUs. Jets and parts are much cheaper than they are for SUs or ZS units, and they are much moreeasily available through better bike shops.

The only slightly awkward bit in the installation was the choke hook-up.The carbs each have individual plunger chokes, which I linked to the choke cable in the car by means of a dual-carb cable splitter box meant for a Norton Domi. Those are available through classic bike shops, and are notexpensive. You need to do a bit of messing about getting the inner and outer cable lengths right, but it is simple once you get at it.

I've messed about with SUs for the best part of 40 years, but the only reason I'd go back to them would be if I were doing a complete concours resto. For a daily driver or a brisk hobby car the Mikunis are light years ahead of the game.

KH

 

Not Kevin Hall's MGB, but that of Greg Smith.