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Author Topic:   Distr. Timing or Carb problem.
Flanndad
Member
posted July 28, 2003 10:00 AM
Blew up my Chevy 406 with camel back heads.

Rebuilt with a set of Pro 220cc heads.

Meausered vacuum at idle off of the vacuum port on the front side of the carb, and it measured 5", so I installed a 2.5 power valve. (This is on a Holley 4779)

Off of the corner the motor would stumble, about half way down the straight the motor would finally kick in, just in time to hit the brake.

It also ran hotter than normal, about 10 lap's of running the track in , 1st gear about 3,000 rpm's and temp went up to 200 degrees.

I believe timing was at 36 degree's but I had a mark on the tappet cover to line up the vacuum advance with and I noticed it was a little bit more counter clockwise from the spot.
I have to recheck the timimg tonight,

Question if the timing is to far advance,
1. Will it make it run hot?
2. Would it run better after the rpm's climb?



avila
Member
posted July 28, 2003 11:05 AM
you might have two problems.

did something happen to the block when
the Chevy 406 with camel back heads blew
causing the higher temp?

what do the plugs tell you, are you leaning motor out?

what is the timing at when you are 3000rpms?

Rick D
Member
posted July 28, 2003 11:15 AM
If you rebuilt the engine you can't use a mark you have scratched from your old set-up to set the timing. Borrow or buy a timing light and set the timing.


Flanndad
Member
posted July 30, 2003 06:25 AM
Rechecked timing, when revved up it doesn't go above 36 degree's.

Pulled the spark plugs and they where a little dark and wet.

Ran car maybe ten minutes, didn't even reach 170*, tried to restart maybe 20 minutes later and had to give it full throttle and hold it there until it started.

To a untrained ear it sounds good at idle, blip the throttle it sounds very responsive and comes back to a nice idle.

[This message has been edited by Flanndad (edited July 30, 2003).]

outlawstock17
Member
posted July 30, 2003 06:39 AM
sounds like you may have a carburetion problem. a little squirt from the accelerator pump and it should start right up. i'd check the float levels and jetting. it might be a little rich.


Flanndad
Member
posted July 30, 2003 09:16 AM
I'm running a 4779 Holley 750 double pumper,
Jets
72 front
80 rear
with a 2.5 power valve.

I have clear sight plugs I did notice the front may be a little high, close to half of the plug had gas in it.
Would this also cause the car to run hot?
I can ubderstand the full throttle starting, need more air in there.



Raz_900
Member
posted July 30, 2003 01:44 PM
Stumbling off the turns generally points to a rich carb problem. The black plugs add to that theory. Maybe change to a higher PV, back off the jets or slow the secondary engagement. We're forced to run a 3310 (vac. sec. 750) on a 355 and use 75 primarys and have the secondary metering plate drilled to be about 78's. We run the 3rd from softest secondary spring and it works pretty well (that spring opens the secondaries fully around 6900 I believe). Trying the next softer spring made it bog.

Get the fuel mixture in line, then track down the temp. issue. It may be unburnt fuel making it hot. Hard to say.


[This message has been edited by Raz_900 (edited July 30, 2003).]

[This message has been edited by Raz_900 (edited July 30, 2003).]

Flanndad
Member
posted July 30, 2003 03:14 PM
Hey Raz,
I'm not a carb guy by far, but I was thinking about changing the primaries back to the 70's.

But I'm wondering about the secondaries, how would I figure out when they are opening, and how do you know that your's are fully open at 6800? You can't idle/rev it that much in the driveway.

What rpm should they be starting to open at?

Raz_900
Member
posted July 31, 2003 01:23 PM
quote:
Originally posted by Flanndad:
Hey Raz,
I'm not a carb guy by far, but I was thinking about changing the primaries back to the 70's.

But I'm wondering about the secondaries, how would I figure out when they are opening, and how do you know that your's are fully open at 6800? You can't idle/rev it that much in the driveway.

What rpm should they be starting to open at?



As for the opening point, that's the guesstimate from the Holley spring kit and info from a Holley insider. I worked as an engine machinist for a guy that worked for Holley back in the early 70s. He still did consulting work a few times a year for them up til he closed up shop and went to work for Crane. He's now the Director of R&D for cams and valvetrains.

Anyway, he always told me that a vac. secondary carb would be smoother than a mech. and almost always faster. Assuming you get the spring in the right general area (want it to open far enough, but not too far) the vac. of the engine will tune the rate the secondaries open for you. A mech. carb puts all that on the tuner, which is pretty **** hard. To be honest, the way they did back in the 70s was to put a plexiglass hood on the car with 2 handles on it. Then they'd take turns riding on the hood of the car watching the secondaries open under WOT. Swear to god, that's what he said. A dyno didn't represent true driving conditions, so they did it in a car. He helped me setup our first car and I just duplicated it on the 2nd car.

On our 355s, he figured having them starting to come in around 2800-3000 would be just about right. And with the springs we use in the secondaries, that's where we are. It's pretty much a trial and error to get it bog free. We tried the next softer spring (open by 5700) and it bogged like **** off the turns. He also figured that the 355 wouldn't need full secondaries in our rpm range (4000-62-6500) and that's what happened too. Your 400 will need a little more than ours, but it might be faster if you slow the secondaries down so they just open all the way at like 90% throttle.

Maybe try a PV 1 or 2 points higher. It might be opening too soon before true WOT. If your carb doesn't have them already, you could have dogleg boosters put in. This will increase airflow about 40cfm and lean it out a little. The dogleg boosters are less of a restriction in the venturi's compared to straight boosters.

[This message has been edited by Raz_900 (edited July 31, 2003).]

dirtracer7
Member
posted July 31, 2003 05:43 PM
Hey Flandad haven,t heard from you for a while.Sounds like your back on the track.If you want to check your vacum you need to warm it up good and check manifold vacum not the carb. Mine runs 7" at an idle so I run a 5" power valve. warm it idle it check it.Good luck racin. Also when I changed my exhaust and intake I had a problem with reversion.which had similar symptoms as you describe if you put it down at the flag it would stumble then come to life down the straight i cured or masked mine with a carb spacer.

[This message has been edited by dirtracer7 (edited July 31, 2003).]

Flanndad
Member
posted August 01, 2003 08:22 AM
Hey dirt racer, how's your racing going?

I switched to Winston Motor Speedway a 3/8 mile track north of Muskegon.

Planned on racing this weekend, but lost the fuel pump in my tow vehicle, probally won't make it.

Decided to try the 3/8's, seeing this season is lost for any point's.
Just getting ready for the special shows.


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