• Hello Guest, welcome to the initial stages of our new platform!
    You can find some additional information about where we are in the process of migrating the board and setting up our new software here

    Thank you for being a part of our community!

Budget carburetor choices for hot engines

buzzboy said:
It seems like all of the carb options flow WAY more CFM than this engine will take. Is that a normal thing with carb sizing? Shoot, even the well hated 32/36 flows 300cfm, which is theoretically ~100 more than this engine will need.
Click to expand...

ITB or multi-carb setups need more CFM capacity than the motor since the CFM of the induvidual throttles are only used on one cylinder and that cylinder only flows on its intake stroke. so even with a combined 1000 CFM worth of carbs they only add up to 250 CFM since its a 4 stroke motor and the intake is only 1 of the strokes.

if the motor runs full throttle for 1 minute then it is only sucking on the carbs for 15 seconds.
 
Last edited:
JohnMc said:
I noticed a fair amount of difference going from a pair of reneedled (properly, thanks to Rhys Kent) HS6 carbs to a pair of DCOE 40's. Very large difference in the higher RPM ranges. And much improved throttle response.

That's with a 2.1L B20 with plenty of supporting mods, though.

I think there are a few more variables besides the plain CFM. The downdraft Weber needs an ugly looking intake manifold that doesn't flow as well as a pair of DCOE's with 4 straight shot intakes. The longer runners on the Weber might help some with low RPM torque, but I think the DCOE's really start to make more power from the mid range on up.
Click to expand...

I think the associated manifolds have a lot to do with it. SU manifold is short and has the wrong pairing. Weber down draft has that damned 90 degree bend. Not only restrictive but must have terrible wet flow characteristics. It needs to be heated to prevent fuel from pooling up under the carb.

Like you said before, the DCOEs probably has terrible atomization, but that goes away once the throttle is WFO at higher revs.
 
carbonmike said:
I think I have first hand experience with all of the major carb options on a B18, other than SUs. My 122 came with a set of Mikunis.

The Mikunis work well. Rotation is not an issue, however they are flex mounted and designed to work with cables an not a linkage. The throttle linkage will push on them, causing the flex housing to bend, no matter how tight the hose clamp is. This causes two significant issues. The biggest being that the distance between the carb and intermediate throttle pivot will change, and can cause the linkage to go past a stability point and stick WFO. The second problems is that the damn carb can work itself lose and fall off. I solved that problem with a ghettotastic nylon link that holds the two pivots together.

IMG_20151112_190217799_zps5vkdip8z.jpg


As I said, the Mikunis work well, but not perfectly. Tuning is easy but tuning options are limited. I go from rich to lean between cruise and 25% throttle. That is all in the needle. There are a few needle options but the only difference is in the OD of the flat parallel section that really only comes into play during idle. All the needles have the same profile in the tapered sections. As a result I run around 11.5 AFR at cruise and get only 20 MPG.


I replaced the crappy weber downdraft on the 142 with a set of DCOEs. These are infinitely tunable and sound amazing. I have modern 152 models. They have an issue with the idle circuit that requires modifications for daily driving. If you go this route spend the extra money and get the 152G models, weber corrected the problems with the 152.

The big difference between the Webers and Mikunis is obviously the tunability of the webers. However the Mikunis are dead dog reliable, engine starts up immediately every time. The DCOEs always start, but it may take some fiddling with the choke when cold, and several seconds of cranking when hot. If you are interested in Mikunis do not bother dealing with John Parker. All he does is replace a pivot shaft, something anyone with a drill press and a pin punch can do.

Ignoring $$$, for a race car DCOEs are the better option.
Click to expand...


This is a good bit of information, and some clever problem solving with the Nylon spreader/holder.
 
burstcurse said:
ITB or multi-carb setups need more CFM capacity than the motor since the CFM of the induvidual throttles are only used on one cylinder and that cylinder only flows on its intake stroke. so even with a combined 1000 CFM worth of carbs they only add up to 250 CFM since its a 4 stroke motor and the intake is only 1 of the strokes.
Click to expand...

Does that even matter with a 4 cylinder engine? There is only ever 1 cylinder on the intake stroke. One carb for the whole engine would be pretty much flowing the single cylinder consumption rate all the time. ITBs will flow the single cylinder consumption rate about 25%, and does nothing the rest of the time. A single intake port is hopefully flowing more than 180 degrees of rotation, but I don't think the sum of the flow at any overlap point would be more than the peak flow of a single cylinder.
 
Yes absolutely, Intake manifold design really determines the carb requirements. 4 large carbs on separate runners may work well despite looking like overkill based on flow numbers, but 4 large on a plenum shared with all 4 cylinders would be over carbed since the flow to the carbs would be smooth and continuous, rather than pulsed and interrupted.
 
Yeah, that SU intake port pairing is very problematic in terms of flow. I've seen some SU fuel injection conversions where they gut the dome and add an injector, that's very much a problem on a Volvo with that off beat intake flow. When do you fire the injector?
 
JohnMc said:
Yeah, that SU intake port pairing is very problematic in terms of flow. I've seen some SU fuel injection conversions where they gut the dome and add an injector, that's very much a problem on a Volvo with that off beat intake flow. When do you fire the injector?
Click to expand...

You would have to fire sequentially and only when an intake port is flowing or cylinders 2 and 3 will be super rich and 1/4 really lean.

In other words, it would be a lot easier to have 4 injectors than to get that setup to work.

That also makes me wonder about cylinder to cylinder mixture ratio even with carbs. When cylinder 2 is pulling fuel and air, then flow switches over to cylinder 1, but the air will accelerate quicker and that initial flow will be lean, while cylinder 2 is getting extra fuel. However, cylinder has to accelerate the mixture from no flow so maybe it all balances out.
 
JohnMc said:
Yeah, that SU intake port pairing is very problematic in terms of flow. I've seen some SU fuel injection conversions where they gut the dome and add an injector, that's very much a problem on a Volvo with that off beat intake flow. When do you fire the injector?
Click to expand...

Right since it'd be crazy to have the injector make 2 pulses so close together, and could be even harder to have an extremely long pulse to squirt both cylinders at once... That was why I had mentioned the the dual injector Ducati TB's, you can correctly fire injectors (with the injector pointed down the corresponding throat, but the pulse pairing is weird, and reverb is kinda weird with the 1-2 and 3-4 "plenum"... My MS conversion, even though the injectors were right behind the valve, there was some weirdness (extremely lean) when I swapped from dual TB's to the Djet plenum.... I summed it up to the wall wetting not getting blown back into the other cylinder
 
I had looked at the SU based fuel injection but ran into this exact problem. The only way I think I could fix it would be to weld injector bungs into the SU manifold and then use throttle bodies in place of the SUs.

Are the Weber manifolds all that bad?
 
Last edited:
Cheaper/easier to jsut slap an injected head on it and use D-Jet injectors. Then just use the un-gassed SU's as throttle bodies.
 
carbonmike said:
Did you notice much difference when you put on the FI manifold?
Click to expand...

I noticed that I had to increase my PW by about 15-20%, and throttle response stayed pretty close to the same. Basically what I thought was that by increasing individual runner length by 4 inches I wasnt getting any reverb from cyl 1-2 and 3-4 which required a bit more fuel to each one individually.
 
buzzboy said:
Everybody with Mikunis seems to be using the HSR series. Would there be a reason to stay away from one of the other Mikuni carbs? They make a few others with CFMs that would match up well.


Or, I could do this setup
Click to expand...


What line were you thinking of? Not completely familiar with every model but most don't seem to have an accel pump, which would be a problem.

PHHs would be awesome but are rare and more expensive than webers.

RS line seem to be ITB and would require a custom manifold.
 
cwdodson88 said:
Are you still thinking individual or dual at this point?
Click to expand...
I have no idea. I'm reading a lot from every avenue that I can find and have probably overloaded on setups at this point. I already have a DCOE manifold and a Dual SU manifold but I'm not opposed to selling them for something else. Thoughts so far:

Try SUs and pray it will idle
Go against Carbonmike's advice aud use a Weber 38/38(some people like them)
Go way over budget with ITBs
Run two big(~45mm) bike carbs, be they Mikunis or something else
2bbl "american" carb

carbonmike said:
What line were you thinking of?
Click to expand...
I am now reading that only the HSRs had accelerator pumps. Oh well.


There is still more internet for me to read and I'll probably have more stupid ideas by tomorrow.
 
buzzboy said:
I have no idea. I'm reading a lot from every avenue that I can find and have probably overloaded on setups at this point. I already have a DCOE manifold and a Dual SU manifold but I'm not opposed to selling them for something else. Thoughts so far:

Try SUs and pray it will idle
Go against Carbonmike's advice aud use a Weber 38/38(some people like them)
Go way over budget with ITBs
Run two big(~45mm) bike carbs, be they Mikunis or something else
2bbl "american" carb


I am now reading that only the HSRs had accelerator pumps. Oh well.


There is still more internet for me to read and I'll probably have more stupid ideas by tomorrow.
Click to expand...


The problem I had with the 38/38 was probably due to the fact that a run of the mill B18 was not big enough to get the primary circuit to kick in before the progression circuit stalled. A B20 or well built B18 could be enough to get the primary to kick in early enough for it to work well.

What kind of DCOE manifold do you have? If it is the shorty version that will fit DCOE 45s and fit in the fender wells of a 122 I would be very interested. could offer of a variety of carbs with an appropriate amount of cash.
 
You must log in or register to reply here.
Back
Top