triple mikuni problems in autocross

[lila725]

Looking for some advice. Last night at autocross, it seemed that every time I exited a tight corner, my car was loading up and missing on acceleration. When done with my run it wasnt until i rev the crap out of it and blew it out that it was running good again. Ive never had this issue street driving no matter how aggresive only when taking slower turn after turn like in autocross.

Its a 73 240 with triple 44 mikuni, isky camm, 500/300, stroker crank 3.1, t5 trans, motorsport header, kn filters, act clutch, light flywhell and dampner, msd ignition etc..

Is this something I will have to live with or is there a problem with the floats or jets of the like. I dont have alot of expierence with mikuni so Im a little lost in the operation of these carbs. Thanks

[robftw]

I know nothing about mikunis however two of my friends both had this problem, the first friend had a bad vaccum, the second friends jets were too small.

neither car was a datsun but it couldnt hurt to check

[jmortensen]

There was a very good post here by tholt29 about how to fix this problem about 5 years ago. First thing is float level. Get it set correctly according to the manual. The next thing is the vents. I believe it's in right handers particularly that the float bowl vents can allow gas to flow out and right over the jets. Tom showed how to JB weld the vents shut and then drill a hole in the cover and vent out the top. I tried to find this old post and couldn't. I used to have the pictures on my computer, but I lost several hard drives since then and apparently those pics went away at some point. You might try PM'ing Tom, I think Phillip Reith (preith) was also in on the conversation, he might have some pictures stashed away as well.

The carbs don't have a lot of adjustments, but still there aren't a lot of people that can really tune them well. I'd start with a manual (I think you can download the Mikuni manual from hybridz.org in the download section) and the How to Modify Your Nissan/Datsun OHC Engine book by Honsowetz. Put an O2 sensor on the car so you can see what is happening in real time instead of trying to read plugs, and just keep screwing around with them. Wolf Creek Racing has jets if you need them. From my own experience I would say that the most annoying thing to try and tune out is the flat spot between the pilot jets and the mains, at about 2500-3500 rpm, especially when you punch it from 2000. My first inclination was to go bigger on the pump nozzle. IME that didn't help very much, what helped more was to fatten the pilot. Cutting the venturi size down works too, but then you lose some top end.

Edited July 10, 2011 by jmortensen

[steve91tt]

The accelerator pump is adjustable. There are three holes on the pump rod into which the cotter pin can be placed. Higher hole...richer on transition. I would try a different hole and see what happens to the the transition.

[Travel'n Man]

I would call Todd - since this is your first post and we do not know if this is a new carb to a current engine or if you purchased the car with this set up. Was this set up for street driving and now you are autocrossing with higher fuel needs and higher RPM's.....at this point nobody knows. Do you have extra jets? What is your current set up of these carbs? We just don't have enough info. Todd has all the parts and knowledge you could need on these carbs - have you read the Mikuni manual - that would be a great starting place to familerize yourself with your set up.

I would call Todd first thing before I turned a screw - but at least do a little reading and research before calling him - IMO.

Todd Walrich

Wolf Creek Racing

516-635-6775

Twalrich@wolfcreekracing.com

www.wolfcreekracing.com

[lila725]

Thanks for the help guys. Funny how you mentioned right handers, I thought that was just in my head but it is worse with right handers. I pulled the plugs and the first4 seem to be fine while the last 2 seemed to be a tad on the wet side so im under the assumption that the problem is on my last carb. I will download the manual and mess around with them and give wolf creek a call as well.

I bought the car from a freinds family as he had a sudden hard attack and past away. I used to work with him and we built the car together and he used it primarily for autocross and road type racing. When we set it up it never had any problems at all although I wasnt really around when he did the carbs. I dont know how long the car was sitting before they called me and told me the news but I went to the house and the car was pretty dusty and the tank was empty, (thankfully). I filled the tank put in a battery and it fired right up and drove it home. I gave it some tlc and put some miles on it and it has been flawless untill now. Like I said driving home from the autocross It was fine so I think th gas spilling out the vents sounds like it may be the issue.

[jmortensen]

Wet plugs probably don't have to do with this problem. I would expect the plugs would dry off as you got back to the grid, etc. I would think the wet plugs on one carb are more indicative of a carb out of sync.

[gnosez]

The secret, and there is a secret, to getting these carbs to handle hard turns (mostly rights) is to send them off to Todd at Wolf Creek, Dave Rebello (call first and see when you might be getting them back) or find someone who races that has them and see if they will mod them for you.

While there may be other things that need correction, Jon was spot on about the vent hole. However there are a few other things you must do as well. It's not pump rod location or jets.

I'm not trying to be a jerk about not publicly responding to your post but I've learned the method to overcome this problem is a closely guarded secret amongst the racing brotherhood and even then most of them say "why would I tell my fellow racer how to go as fast as I do".

I hear switching to EFI solves the problem but since my street car has a set of 44mm Mikunis and our race car a set of 44MM taper bored to 50mm Solexes (from the 260 BSR car) I wouldn't know.

[lila725]

Ok here is what I came across. I contacted todd at wolf creek and he said first thing to do is check the fuel pressure, (It needs to be 3psi +/- .5, and the floats to be set close to 14 mm. I have a holley pump 9 psi and a regulator with a in line pressure guage after the regulator. Well the guage is bad and reading low especially when hot so I had way to much fuel pressure. Got a new guage set it to 3 psi and set the floats to 13mm per Todd, (He said close to 14mm is good evem though manual calls for 12.5mm. Resynched the carbs and yes last carb was out a little, changed the oil and went and beat the crap out of it and it seems pretty good. Next race is in 2 weeks so we will see what happens. If it happens again im pulling them off and sending them in to todd, Like you said I wont ask what he is doing but I know it will work. Thanks again guys for your help. Ill post some pics soon so you guys no who i am and can see my car.

[spitz17]

He said you needed 3psi +/- .5 on a 3.1L Stroker?

Just clarifying because mine is set to 4.5 psi, and I have had similar problems as you describe.

[lila725]

Yes he said 3psi +/- .5 . I had mine at about 5 psi and i have lowered it to 3.3 psi at norm operating temp and it is running beautifully. Double check your floats, mine ar at 13mm. I have not done another autocross yet with these settings but driving around on the streets and taking some hard corners it has not yet given me a problem