360: timing/mixture/vac help needed.

David Womby

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My engine:
1978 Monaco 360 engine.
Mopar Performance Cam 4452761.
Mopar Performance Intake Manifold P4529116.
Piston rings 4.030"
Pistons unknown
Exhaust valve 1863
Summit Racing Carb SUM-M08600VS. 600cfm
Distributor Pertronix PNX-D141701.
Coil Pertronix PNX-45001.

The exhaust smells of gas. So I thought I'd lean out the idle mixture. I use a vacuum gauge looking to set the idle mixture at maximum vacuum I can get. Idle speed is set at 1050 in neutral. I could only get 17in of vacuum with a slight flutter between 16.5 and 17.5. Is that right for this engine? Thanks.

David
 

rumblefish360

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Hey David.

Does the engine idle lower than 1050? Can you bring it down to 9 or 8 hundred?
Did you or do you have a jet kit?
What are the plugs gapped to?
The flutter can be a few things.

That cam, as most aftermarket cams for performance are cut on a 110 LSA. This tends to lower the vacuum some. I think what you have is pretty good. IMO, once you lean out the mixture and turn down the idle it’ll be fine.
 

MIKESPOLARA

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After you adjust the idle speed down the vac will probably drop to about 14-15" which is normal for a performance cam-especially one that is cut at 110 degrees

Vacuum depends alot on how well the engine was built
 

David Womby

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Hey David.

Does the engine idle lower than 1050? Can you bring it down to 9 or 8 hundred?
Did you or do you have a jet kit?
What are the plugs gapped to?
The flutter can be a few things.

That cam, as most aftermarket cams for performance are cut on a 110 LSA. This tends to lower the vacuum some. I think what you have is pretty good. IMO, once you lean out the mixture and turn down the idle it’ll be fine.

Thanks. I can try dropping the idle a bit but last time I tried it at 900 in neutral it was down to 650 in drive and not too happy about it! I don't have jet kit and believe the carb has the standard ones that Summit fit which are primary 67 and secondary 73.

I can't remember the plug gap - I think it was .45 but will have to check. I need to check what the plug brand and type are too.

After you adjust the idle speed down the vac will probably drop to about 14-15" which is normal for a performance cam-especially one that is cut at 110 degrees

Vacuum depends alot on how well the engine was built

OK. Thanks. The engine was rebuilt by Muscle Motors, Lansing but back in 1991!!! I don't think it's had many miles on it since it was rebuilt but have no way to know.

David
 

Dan64

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What is your initial timing and is it steady at idle?. A large drop in rpm could possibly mean that the springs in the distributor are too light and are already slightly advancing the timing at idle. You could put a timing light on it and watch as a helper puts the car into gear and make sure that the timing doesn't go down. Not enough initial timing could also make the car stink like gas at idle.
 

David Womby

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What is your initial timing and is it steady at idle?. A large drop in rpm could possibly mean that the springs in the distributor are too light and are already slightly advancing the timing at idle. You could put a timing light on it and watch as a helper puts the car into gear and make sure that the timing doesn't go down. Not enough initial timing could also make the car stink like gas at idle.

Thanks. I'll check the timing tomorrow.

David
 

MIKESPOLARA

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Convertor may also be too tight for performance engine (need higher stall)
 

David Womby

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Convertor may also be too tight for performance engine (need higher stall)
Good point but the transmission was provided by Muscle Motors too as far as I can tell from old faded receipts. I can't make out anything except that it's a 1962 727. I'd hope it was suited to their rebuilt 360.

David

Favid
 

MIKESPOLARA

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I think you have a few things going on here. Not enough initial timing because when you have idle up high the dist is already up on it's curve so when you lower idle the distributor go's back to initial setting and you don't have enough initial timing PLUS the convertor is too tight. Once you get the timing issues sorted you'll find out if convertor is too tight because it'll run good but any time you go from park to drive and there's a big drop in RPM it's your convertor-assuming you got the timing issues worked out.
 

David Womby

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I think you have a few things going on here. Not enough initial timing because when you have idle up high the dist is already up on it's curve so when you lower idle the distributor go's back to initial setting and you don't have enough initial timing PLUS the convertor is too tight. Once you get the timing issues sorted you'll find out if convertor is too tight because it'll run good but any time you go from park to drive and there's a big drop in RPM it's your convertor-assuming you got the timing issues worked out.

Ok. I'll report back after checking timing tomorrow. Thx.

David
 

David Womby

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OK. The findings this morning on a hot engine.

I checked 2 plugs #1 and #4. They are Champion RN14YC and both look to have nice tan ceramics and are gapped at .035" but there is soot around the edge of the metals threaded part.

#1
Plug #1 (Copy).jpg


#4
Plug #4 (Copy).jpg
Plug #4 soot (Copy).jpg


I disconnected vac advance to the distributor and plugged its port on the carb.. I think I did the timing measurements correctly but am not 100% sure.

Here's timing measurements I saw.

Initial timing @900rpm 10BTDC
Total timing @2500rpm 25BTDC

but when I dropped back to 900rpm, timing showed 7-8 not the 10 I saw earlier. The total seems low to me?

At idle of 900rpm, the best I could do on vacuum was a wobble between 13 and 14in..

Any thoughts?

David
 

Dan64

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Those numbers are too low to me. I would leave the vacuum advance off and plugged for right now until you get a handle on everything. Set your total timing to 34 ish and that will of course also move your initial ting up as well. It's working a little backward setting total first but i would do it this way so you can figure out how much mechanical advance is built into the distributor and to be safe for the engine. Once you have your total timing set to 34ish at about 2600-2800 ( you can play with the curve later) make sure that if you rev the motor higher than that 26-2800 the timing doesn't go any higher than the 34. Rev the motor up to 4500-5000 and make sure 34 is max. Once you got that done let the motor idle around 850-900 if it will and check the timing at idle and write down that number.You want the timing rock solid at idle meaning when you put the car into gear the timing doesn't drop below that number you wrote down. If it does that means that the springs in the distributor are too weak allowing the weights in there to bounce around or actually introduce more timing before it's supposed to. If the converter is too tight when you put the car into gear the rpm's will drop a lot but you will feel a very noticeable pull ( you'll have to press down very hard on the brake pedal to not allow the car to move= too tight of a converter.
 

rumblefish360

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I set my initial at 18-20, all in by 34* with Edelbrock heads. AKA closed chambered heads. Plugs look OK for what you described in how and what your running. Vacuum readings are about right. Don’t get so hung up on the vacuum reading.

The big rpm drop is weird.
 

David Womby

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Again with hot engine and vac advance disconnected and its port on the carb capped:

Set total timing at 34BTDC @ 2800rpm. Revved higher but advance did not increase further.

Dropped back to idle and advance dropped to 10-11BTDC @ 850-900rpm. Idle rpm not totally stable but engine sounded good.

I didn't have a helper so I couldn't read timing when the car was put in Drive so that will have to wait for another day - maybe this weekend - but I reconnected the vac advance went for a brief run.

Surprise: idle in neutral 1200rpm and 850rpm in drive. A definite thump putting it from neutral to drive but that's because the idle somehow was too high again. I'll fix that next time. Driving was fine, maybe running a bit smoother than the old timing. No hills here but I couldn't detect any odd noises pulling away from a stop or when using kickdown.

A bit hard to restart when hot and also a couple of hiccups on shutdown (like run on) but I had forgotten it was starting to do that the other day after installing the new pump. Not sure why that would be.

David
 

Dan64

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Ok, I think the springs are too weak. If your idle rpm seems to change on its own that could be that the weights are advancing the timing at idle, you most likely need a slightly stiffer spring in there. You don't want it to advance until around 1200 rpm or so on a car idling at 900. If you don't have a helper you can check for steady initial timing by watching the timing mark with the timing gun as you hit the carb's accelerator pump ( not the throttle ) this shot of fuel with no air ( throttle blades are closed) will make the engine want to bog down- timing should not go down with rpm at this point. The hard start sounds like too much timing but your 34 and 10-11* not not high at all.
TDC should be verified with a piston stop tool, it's pretty cheap and easy to do and i'm sure there are plenty of videos to show you how. Sometimes this stuff is like a ball of string and you just need to find the two ends for it all to come together.
 

David Womby

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Ok, I think the springs are too weak. If your idle rpm seems to change on its own that could be that the weights are advancing the timing at idle, you most likely need a slightly stiffer spring in there. You don't want it to advance until around 1200 rpm or so on a car idling at 900............................................................................

................................Sometimes this stuff is like a ball of string and you just need to find the two ends for it all to come together.

LOL - I get the piece of string bit and expect to be chasing the 'right' tuning for a while yet. I will experiment with it a bit more before going after the distributor springs but probably not until the weekend. Thanks.

David
 

David Womby

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I experimented a bit today to see if I could get rid of the hard starting when hot. I ended up at 9BTDC initial @950rpm and maxing at 31BTDC @2800rpm and above. It seems to make hot starting ok but hasn't cured the slight dieseling. Here's a video. https://youtube.com/shorts/JImxGbYFjN8?feature=share

Thoughts? If I drop the idle lower, the car stalls when put into Drive sometimes. Any help will be gratefully appreciated.

David
 

David Womby

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I updated this thread title because the carb spraying gas on shutdown in yesterday's video is pretty scary. Although I can't work on it today, I really need some more help on how to get this thing tuned properly, guys, please.

David
 
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Don Frelier

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I think it's running way too fast, that's the big problem here.
Just as a test try 20- 25 Degrees BTDC and slow down the idle as much as possible and tune it at idle in gear when warm with the air cleaner attached.
You may need to drill 1/8" holes in the throttle plates so they are closed more at idle.
If that helps then you'll need to cut down the amount of mechanical advance to get your maximum advance in the 35 degree range.
But that's a separate project.
 

WileERobby

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Some excellent advice on the past few posts. You may not know, but is the cam set up straight, retarded, advanced ? As a prior posted noted, setting ignition timing backwards is something I routinely did. Set total, then adjust advance & initial to suit.
 
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