2020??????

BajaBoy7 · Post by **BajaBoy7** » Fri Jun 28, 2002 6:29 am

I have just finished building a 2020(?) 76.4x90.5 with 5.500 rods and CB044 P&P heads with 7.8:1 CR. I am running a web cam 109 straight up and single 44 carb with 28 vents on a 1 5/8 manifold. I just can't seem to get it to run right. It stumbles off idle until it gets going then runs strong in the mid but wont pull past 5K RPM. I would like to stay away from duals cause its in a Baja. Is it my jetting, carb, CR, or cam? Thanks in advance for you wisdom.
Kevin

UncleBob · Post by **UncleBob** » Fri Jun 28, 2002 7:20 am

What jets are you running? For one thing, I believe your venturis are a little on the small side.

BajaBoy7 · Post by **BajaBoy7** » Fri Jun 28, 2002 7:38 am

Currently I am running 65 idles 145 mains with 160 airs I belive. I have tried almost every main jet from 125 to 165. I went from f11's to f7's which made it stumble alot less. Also when crusing at about 2000 Rpm it surges. I think my large manifold is causing the down low problems. Should I try 32mm vents?
Kevin

[This message has been edited by BajaBoy7 (edited 06-28-2002).]

mchn13 · Post by **mchn13** » Fri Jun 28, 2002 10:22 am

unfortunately i don't know enough about jetting,but i would go to 36 or 38mm vents,and bump the cr to8.5 or more.

rajay · Post by **rajay** » Fri Jun 28, 2002 12:47 pm

the 28 venturi should be replaced with 36's, your pump jets should be 80's, your idle's might be okay at 65, if you go back to the f11's and run your 160a/c's look for 170 mains.

BajaBoy7 · Post by **BajaBoy7** » Fri Jun 28, 2002 12:53 pm

WOW Ok 36's it is. Sounds like I need more fuel huh? Thanks and I will give it a shot.
Thanks
Kevin

Post by **Marc** » Fri Jun 28, 2002 2:15 pm

Bigger chokes (venturii) will give you more air and more RPM, but when you go to larger chokes the vacuum signal won't be as strong and you'll need bigger mains to compensate.
If it ran better down low with F7s it must want more fuel - the F7 is a "richer" tube by nature because of its smaller diameter and lower hole placement, leaving more fuel in the well around it to be available on acceleration.
I wonder how much of your problem might be due to fuel condensation in the manifold? That's a long trip from carb to heads, and all of the single IDF center sections I've seen look too big to maintain decent flow velocity at low speeds. You might consider improving your manifold preheat and/or rigging a warm air pickup to bring some hot air to the aircleaner.
You should be able to change the pumpshot volume by moving the pivot of the pump arm, too (presumably you'll need more than a dual-carb setup would).
I congratulate you on a sensible C.R. choice; do you have an adjustable cam gear? A couple degrees advance could sharpen up the low-end response.
I haven't run a single IDF on anything so can't offer any specific numbers for that, but I've never had to run F7s in any dual IDF application. Your big engine is pumping the same CFM through one carb at ~2900 RPM that each of my carbs did at 7200 on a circletrack 1641, so there's probably not much point in trying to make comparisons, but FWIW I was usually around 165-170 mains and 195-200 airs with 36 chokes.

[This message has been edited by Marc (edited 06-28-2002).]

BajaBoy7 · Post by **BajaBoy7** » Fri Jun 28, 2002 11:26 pm

Those are what I suspected. I run 91 pump gas and did not want to have to go thru the race gas thing again thats why i went to 7.8:a CR. I'll have to find some bigger venturies and try those.
Thanks
Kevin

BajaBoy7 · Post by **BajaBoy7** » Mon Jul 01, 2002 11:46 pm

OOPS Sorry guys they are 36 mm venturies. I think i'll go back to the f 11 with bigger accel. jets