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  #1  
Old 11-11-2007, 08:35 PM
81mirada 81mirada is offline
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Default missfire and then it takes off (360 c.i.d)

I have a 360 with a weind intake,j-heads,holley fuel pump, msd coil ,eldrebock carb., headers,cam I think its 292 with 488 dur.accel spark plugs
stock distrabuter, spark plugs gapped at 46, o. k my proplem is car turns
on great but when I take off it boggs and then it takes off.but if you try to stop like put the brakes it turns off.my timing is correct at 16degress btdc.
my carb is a 750 but I installed smaller jets . can it be the carb or bad spark plugs. If it is ran in park at a high idle it revs up and then missfires start
at time. can it be the gas 87 octane? Any help will help .
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Old 11-11-2007, 08:49 PM
dodger1 dodger1 is offline
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Vacuum leak? Is your vacuum advance hooked up correctly? (to ported, not intake, vacuum source) Applying the brakes should have no effect on engine operation unless you already have a vacuum leak to start with.
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Old 11-11-2007, 11:42 PM
BigBlockDude BigBlockDude is offline
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Sounds to me like the carb is messed up. Has this carb worked on a different engine, or is it new? Also why did you install smaller jets?
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Old 11-12-2007, 12:30 PM
81mirada 81mirada is offline
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I installed smaller jets because I was told a 360 requires a 650 carb. Thinking of installing a new thermoquad I bought. My 750 carb was bought at a swap meet. Thinking of buying a avenger(holley) or do you think thermo will be O.K?
I dont know if the 750 carb ran on any other engine.
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Old 11-12-2007, 02:58 PM
dodger1 dodger1 is offline
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If you have the Weiand spreadbore manifold, the TQ would be my choice (I run one now), if squarebore, then the Holley or Edelbrock would be it. I'd try and find that bog first, before throwing $$ at it.
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Old 11-12-2007, 09:09 PM
BigBlockDude BigBlockDude is offline
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Quote:
Originally Posted by 81mirada View Post
I installed smaller jets because I was told a 360 requires a 650 carb. Thinking of installing a new thermoquad I bought. My 750 carb was bought at a swap meet. Thinking of buying a avenger(holley) or do you think thermo will be O.K?
I dont know if the 750 carb ran on any other engine.
The jetting change is not necessary. You just leaned it out alot. Probably needs a rebuild and would be fine.
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Old 11-15-2007, 06:03 PM
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pcrmike pcrmike is offline
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Ditto BBD. Sounds like my AFB did with stock jetting. Fattened it up and all's well. ;-)PCRMike
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Old 11-16-2007, 05:34 PM
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TrashedCharger TrashedCharger is offline
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I downjet my carburetors, but I'm at about 6,000 feet in elevation above sea level and I need it.

750 cubic feet per minute is too much flow for a stock J head.

A camshaft that has measured lift for 488 degrees of it's rotation is physically impossible on something that only rotates 360 degrees before going back to zero. Do you mean that your camshaft has 292 degrees of duration and .488" lift? Is it .488" intake and exhaust, or are those just the intake numbers? Are you running stock rocker arms with stock hydraulic lifters?

Even with a cam at that stage you are simply opening up too much air into those heads, especially with a smaller jet/larger metering rod. Unless this is a race car that isn't running a choke and metered to run excessively rich with bigger valves, 750 is just too much for a stock 360 block and heads.

If you jet that carburetor back to get the correct air to fuel ratio for it's CFM, you will be dumping fuel out of your exhaust and wasting money and resources. Downjetting does not lower or compensate for the cubic feet per minute rating of a carburetor. It simply adjusts the air to fuel mixture ratio for that CFM rating, but no matter what you do, your carburetor will still be rated at 750 CFM. That is important to understand when tuning an engine. You should always match a carburetor's CFM to the displacement of the cylinders, along with the flow of your heads and compression of the heads/cylinders combined.

650 cubic feet per minute is even a bit on the high end for a stock 360 J head, even with a mild camshaft. but if you do have a 290+ degree duration cam with a choppy idle that is pulling in more air, you are probably in need of about 650cfm. This depends heavily on the measurement numbers from that camshaft. If you overestimate your camshaft's duration and lift numbers, again, you are wasting fuel and hurting performance on all aspects.

Make sure you know what that camshaft is and get a carburetor with a suitable CFM for the flow of those heads. Then jet it for the correct air to fuel mixture ratio for it's CFM rating and for your altitude.

If it's any consultation to you, I know someone with a re-rung stock 360, running stock heads with bigger valves, an edelbrock dual plane performer intake and an edelbrock 650 CFM AFB carburetor on her Dart race car, through headders with shorty mufflers. The camshaft in that car is at 280 degrees of duration and it won't idle under 900rpm.

You should also know that higher duration camshafts, having a lot of overlap, have a hard time idling, due to the lower amount of vacuum that they produce. If you have vacuum operated stuff on your car, such as an A/C heater box, a power brake booster or hideaway headlight servos, you should put a vacuum canister on your vacuum system. It should go between the component(s) and the vacuum source off of the engine. This is to help baffle and regulate the inconsistant vacuum of a high duration camshaft at idle. Otherwise you could kill the engine by using the brakes or other vacuum operated mechanisms on any lower rpm, either in rpm drop or at idle (which is when you are braking, if you think about it).

I would also suggest gapping the plugs a bit closer. You could be blowing the spark out. Factory adjustment is at .035" With today's fuel, I would run them a bit larger, but not the .010"+ more that you have them at now, unless you have some sort of ignition amplifier/ multiple spark discharge system and the flow to support a .045"+ gap.

I would suggest using a 625 CFM Carter/Edelbrock/Weber AVS or AFB, but if that is a true 290+ duration camshaft, you can run 650 nicely. And as stated above, if you have the larger bore intake manifold, I would run an 800 CFM Carter ThermoQuad. Don't be fooled by the large CFM rating of the ThermoQuads. They have very small primaries and have the driving charactaristics of carburetors with a rating of about 200 CFM less, even though they are technically higher CFM.
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Old 11-16-2007, 06:08 PM
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TrashedCharger TrashedCharger is offline
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This reminds me of another vehicle I was working on. My brother had bought a Ford Bronco that was lifted with 35" BFG's, etc for a four wheeler. The guy who owned it before him put a 750 Carter Competition AFB on the 302 that the truck had.

When we went to get it tested for emissions, even with lower jetting, the carbon parts per million were nuts. I think it was somewhere around 15.3ppm, with an allowed 5ppm on that specific vehicle.

I knew that we were going to have issues when we went to the emissions clinic, but my brother insisted on trying to get it to pass first, before we did anything to the carburetor.

We went home and swapped it for a 650 Edelbrock AVS that I jetted two stages down from sea level for our altitude. We noticed an immediate vast improvement in throttle response, power, torque, gas milage and it stopped smoking on sudden acceleration.

We went back to the emissions clinic and got it to pull 4.8ppm on both banks without cataletic converters (which we discovered were missing).

It passed the sniffer test, but failed the visual, so we welded on a pair of high flow cats in place of the glass packs and used a 12" 2 1/2" ID shorty muffler after the cat to make up the missing pipe that the glass packs compensated for.

We took it back to the clinic to get our thumbs up and even though it was just a visual that we had to pass, I asked if we could put the sniffer on it. It went down to 3.9ppm of carbon and we actually gained some more power, because of the corrected back pressure coming from the cat and race muffler, versus the glass packs.

That truck got 17 mpg in the city. Not bad for a tired 302 on a truck with 35" swamper tires! It just goes to show that bigger isn't always better.

We no longer have the Bronco, but we don't miss it either. We can appreciate some of the oddball FoMoCo cars, especially Merc's and Lincolns, but not really enough to continue owning any. I've owned a few economy fords, but nothing that I was planning on keeping. Mostly hand-me-downs and cars that I could buy for $50 that only needed a few hundred bucks to resell for a couple thousand.

I have a soft spot for the Chrysler corperation and for older imports, but I'm no brand loyal snob.

Good luck with your car!
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  #10  
Old 11-18-2007, 04:58 AM
BigBlockDude BigBlockDude is offline
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A 750 is not too big for a 360. Stock or otherwise. I ran a 440 Thermoquad (850cfm) on a stocK 75 360 in a 69 Fury. Ran great.
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