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Author Topic:   Engine Tuning Problems
Dirt Roller

Total posts: 19
posted April 29, 2003 08:09 PM
Ok, here is the deal. I have a 10.5 to 1 engine. Freshly built 383 stroker. I just cant get this sucker dialed in. The engine starts missing at 1,500 rpm on up. It really sounds like crap and on practice night could only get up to 5,200rpm when I am used to racing around 6,400rpm on a weaker engine. I've pulled the plugs and no matter how high of a jet size I go I kept getting a lean condition. Even at 90 jet size it's lean!!! Actually, it really doesnt look lean. It really looks like it is clean or dry or something. I have never seen a plug look like that before. Anyway, I have the total advance timing set at 32 degrees and even there it misses. I have swaped hot plugs to cold plugs and vice versa. Sucker is still lean. Now being that it is a 10.5 to 1 engine I upgraded to a higher octane fuel. Not racing fuel but avgas. A friend of mine told me to use the avgas but now I am wondering if it is this fuel causing the problem. I have tried everything to get this engine dialed in except changing to a different fuel. I dont know anything about avgas but am beginning to wonder if it is junk. If it is not the avgas, then what could it be?

Dirt Forum Champ
Total posts: 564
posted April 29, 2003 08:26 PM UIN: 25419780
it sounds like a bad vacum leak.. or could be a bad carb..

Dirt Roller

Total posts: 19
posted April 29, 2003 09:08 PM
I checked for a vacuum leak at the carb base using starting fluid. There was no change in engine rpm so that's ok. I checked all the vacuum plugs for cracks and they looked good. The only thing I didnt check is for a leak at the intake maniold. I am not sure how to check for this. What do I do?

Dirt Maniac

Total posts: 128
posted April 29, 2003 09:18 PM UIN: 14464409
it sounds like the plug wires may be breaking down, if they are new make sure they are not touching each other. and re run them to make sure they are on the correct plug. and the way to check for a leaky manifold is to spray wd40 on the edges of the manifold while the motor is running and if there is a leak it will make a change in the idle. if you are running hydrolic liffters some may be set to tight and when the rpm comes up so do they. good luck.

Dirt Full Roller

Total posts: 55
posted April 29, 2003 09:43 PM
It could be that your valves are not adjusted correctly. Does the motor
idle ok?

Dirt Full Roller

Total posts: 46
posted April 29, 2003 10:13 PM
i'm a rookie, but i'ld check that carb at this point.

Dirt Full Roller

Total posts: 72
posted April 29, 2003 10:44 PM
Check your power valve size. Lower manifold vacuum requires a lower # valve to work properly.

racer 4
Dirt Full Roller

Total posts: 34
posted April 30, 2003 03:00 AM
if your heads or intake has had any kind of angle milling it is possible that you may have a vacume leak on the under side of the intake manifold take the carb off and you may be able to see far enough down the intake to get a general idea if the intake to head ports line up if you cant see anything obvious then check ignition components like was said in a earlier post good luck. if by chance the ports dont line up use a mr. gasket #106 its thicker and should fixerupok!!!!!!!!!!

Dirt Roller

Total posts: 21
posted April 30, 2003 08:29 AM
Check to make sure you have the correct sparkplugs for the heads you're using.If they extend too far into the cylinder they'll run too hot.It will cause the problem you're describing.I've had this happen before.

Dirt Freak

Total posts: 408
posted April 30, 2003 11:31 AM
Aviation fuel should never be run in a racecar. It is formulated for ultra-high altitude operation at very static rpms, and is a very dry and clean burning fuel. It tends not to color the plugs at all unless the motor is so flippin' rich it won't run.
Your fuel needs to be changed, firstly, and secondly, has someone been inside your carb? The only reason I ask is that NO unaltered gas carb should run and still show a lean condition with jets as big as you say you tried.
Give us an idea of what the history of that carb is, and I'm sure we can fix ya up.


Dirt Newbie

Total posts: 2
posted April 30, 2003 03:07 PM
Check the little things.... fuel filter and especially the fuel pickup line in your tank. Fuel cell foam and other things can get sucked up in there and will explain why it doesn't matter what size jets you put in.

Dirt Full Roller

Total posts: 29
posted May 01, 2003 03:15 AM
Are you running the dorton carb? We had a similiar problem we were running a 350 last year, and we switched to a 400. The 400 would not run with the standard 350 cfm carb. We switched to the dorton carb and it made a huge difference. Our carb was not the best to begin with, but it would be worth a shot.

I am sure that KPLugnut can probably get your current carb going for you though.


Dirt Roller

Total posts: 19
posted May 02, 2003 05:39 PM
Ok guies, I think I got her fixed. Thanks to all of you for your advice. I pretty much checked everything that all of you said. I reset the valves, changed fuel filter, emptied all the avgas and put in premium 93 octane from Amoco (hope it's enough ), put stiffer springs in the distributor, etc etc. But one problem I did come across is that I noticed the fuel pump was leaking fuel through the weep hole. I guess that was a sign that the diaphram had ruptured but not really sure if that is why the engine was missing. Float bowl seemed to still get plenty of fuel. After all that I went ahead and fired her up. She sounds frickin awesome! Not a miss but just a little backfiring from the headers........and that's fine to me. Now, I'm ready race tomorrow night. Thanks again everyone. Bye the way, my carb is a holley 4412 500 cfm. Carb's working fine.

Dirt Freak

Total posts: 230
posted May 02, 2003 06:15 PM
Had the same problem once. Was a bad rotor.

Dirt Roller

Total posts: 19
posted May 04, 2003 11:34 PM
I gave all the avgas to my fiances father who lives on a farm. He put some in a jar and let it sit. Water had seperated from it! 1/2 water and 1/2 fuel. I new it was junk. I told the airport but no refund

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