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Author Topic:   Carb help please!!
g71
Dirt Roller

Total posts: 16
posted January 26, 2005 04:56 AM  
Changed cam in engine to meet 17" vac. rule@ 1000 rpm. Can pull vac. fine, but must run idel mixture screws 1/2 turn out. engine stubbles bad until boosters come in. this remains the same with 750 ,830 ,850 Holley on citgo red 110 race gas.

KPLugnut
Dirt Forum Champ
Total posts: 1019
posted January 26, 2005 07:39 AM  
Sounds firstly like you've been using too large a carb for that cam/motor combo. Nothing larger than a 650 carb would be needed with that small a cam grind, but I'd have to get more info on your motor to be sure.
Next, are you getting accelerator pump shot immediately when the throttle first starts to move or is there a big delay in it?
Don't worry at this point where the idle mixture skrews are set, other than it's good they are consistant from side to side.
Having to have them only 1/2 turn out means you're pulling most of the needed idle fuel from somewhere else, which could be partially due to too large a carb in the first place.
Let us know more engine info and we can sort it out.

KP

g71
Dirt Roller

Total posts: 16
posted January 26, 2005 09:03 AM  
Thanks, for the quick reply. Fuel does start to flow immediately from squirters,but seems a little short lived. Throttle plates have not been drilled. 355sbc flattops.11.5.comp. timed@32 ,fuel 6.5# constant.Engine temp 190@30mins. run time.To start engine You must pump pedal a few times and hold 1/2 open.If at any time You quickly release throttle the engine stalls.

KPLugnut
Dirt Forum Champ
Total posts: 1019
posted January 26, 2005 09:22 AM  
Sounds like there is a problem in the idle circuit of the carb, as well as possibly a blown power valve or leaking one.
The lack of holes in the primary butterflies will contribute to where the idle circuit settings wind up, but first, I'd be checking into where your fuel is going if it's not staying in the bowls, and why it's so hard to start.
Has the carb(s) been built or modified by anyone? The symptoms you describe could be caused by about half a dozen little things being wrong or mismatched in the carb.
Don't worry about a short-lived pump shot, it's not needed after the initial lean spot is taken care of anyway.
with the engine at idle, and throttle closed, is there fuel dribbling from the boosters?

Hang in there, we'll figure it out.

KP

g71
Dirt Roller

Total posts: 16
posted January 27, 2005 06:42 AM  
KP, thanks for help . I have located a carb,that is suppose to be in good running condition.I will try it & give You the results.

g71
Dirt Roller

Total posts: 16
posted February 02, 2005 06:40 AM  
KP ,got a 650 from a friend.I had to do alot of cleaning,parts replacement etc.You are right it runs pretty good now. The holes are so tiny under the power valve is that safe? The frt. has 7.5 valve #68 jets,rear plugged with#76 jets.Is there somethig I could do,toget this better for next weeks practice session? We have a 2400# car, about 400h.p.and race on dirt 1/4 mile ovals.Rpm is 2800 -7000. I certainly thank You for tech advice.

KPLugnut
Dirt Forum Champ
Total posts: 1019
posted February 02, 2005 07:25 AM  
Lots can be done, but most is hard to do in the timeframe you have.
What squirter size and pump cam is on it? How "small" are the power valve channel restriction holes (behind the primary PV)? Try to drill them to .050" or no more than .055" will be fine.
Jetting will be fine to start, but monitor it based on air temp/quality.
How is the throttle response, and is there any delay in the pump shot when you hit the throttle? Have you set the idle circuit up properly (mixture screws, primary butterflies in relation to transfer slots, etc)?

Let me know and we'll get it as good as possible without surgery.

KP

g71
Dirt Roller

Total posts: 16
posted February 03, 2005 06:02 AM  
KP, what a treet to see this carb take on racing form,from your guidance.This is the latest report as you requested.The frt.sqt.#31/brown wide cam ,the rear has #31/pink.The pump shot is a good stream, lasting till frt. boosters are flowing.Transition to full trottle seems a little fuzzy .After warming engine ,setting rpm@1000,adjusting mix screws,I removed carb and measured .035 showing, of the slot. I did carefully drill the 2holes under the pv .053, they were .034. Thanks

KPLugnut
Dirt Forum Champ
Total posts: 1019
posted February 03, 2005 06:56 AM  
OK, 2 pink pump cams will be sufficient for accelerator pump shot (the wide brown I would presume is made for a 50cc pump). And a 50cc pump is not needed on a 4bbl unless it's an alky carb.
You'll be able to drop down to at least a .025-.028 primary squirter to get the pump shot timing re-aligned and get it's duration back, too.
Enlargement of the PV channel restrictions usually allows you to drop down a jet size or two since you're adding more fuel quicker thru the PV circuit now, and you may now be on the rich side which can contribute to the laziness you feel.
Get that crispness back into the carb, and it's as good as you'll get from it without major internal "surgery".

Good luck,
KP

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