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302 Thumpr Cam Won't Idle

pennyduke

Sr. Member
Joined
Jul 18, 2010
Messages
377
Loc.
Stafford
Back at the end of 2016 I mentioned buying a Thumpr Cam for my build. 74 302 .030 over, Duraspark, New Painless Harness, 3G upgrade, and GT40P heads. A few people said to go for it and one person said "DON'T DO IT". Well I should have listened to him. I have a new Edelbrock 1406 carb and this thing won't idle after getting warmed up to operating temperature. I have plenty of torque and top end power but it just won't Idle.


Things that I have tried:


Set the Idle Mixture screws per the Elelbrock's book


Advanced the timing from 10 to 15


hooked the vacuum advance to Manifold vacuum


Changed the location of the squirt rod to the middle hole


tried the choke on different settings


New gas filters and made sure the gas lines are not getting hot anywhere


Engine temp stays at 190

Any recommendations on what to try next? I'm about to head out there now and pull this POS cam out tonight. Thanks everyone
 

Broncobowsher

Total hack
Joined
Jun 4, 2002
Messages
34,833
So you have a new cam and carb at the same time and are looking for the problem?

Knowing you had the intake off as well I would consider the intake manifold gasket as a very good suspect. They can fail and you have a massive vacuum leak. You might be able to cover that up on a cold engine with a lot of choke. Also, any open vacuum ports you forgot to plug?

Manifold vacuum advance causes erratic idle. As vacuum increases, so does idle, which tends to increase vacuum at idle. As vacuum drops, advance drops, lowers the idle which lowers the vacuum. Evil fight.
 
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pennyduke

Sr. Member
Joined
Jul 18, 2010
Messages
377
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Everything on the engine is new except the RPM intake and the GT40P heads which were redone at the machine shop. I used new gaskets for the intake and took great care installing them. I only have 3 vacuum ports on the carb in the front and one on the rear of manifold going to the C4 trans. The big port on the back of the carb is plugged. I tried the 2 front small ports in the front for the vacuum advance while blocking the other. Then I blocked them both and used the manifold port for the C4 and still won't idle when warmed up. How would I find a Intake leak without removing the Manifold?
 

tirewater

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Jan 28, 2011
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You need to describe 'won't idle' better. Does the engine die right away? Will it idle if set at 1k+ rpm?

Did you check the vacuum with a vacuum gauge?

I thought thumper cams had a lot of overlap, and thus very little vacuum at idle. I'd be surprised if there was enough vacuum at low speeds for power brakes and the automatic transmission.
 

68ford

Bronco Guru
Joined
Dec 26, 2004
Messages
2,710
What is the cam specs? Especially what is the lobe separation? Thumper cams typically have tight lobe separation like 108 so they have a lot of overlap making them lope without having huge duration and lift. I run an Isky mega 280 and it is the same. With a stock converter, to keep it idling and not hint when in gear warmed up, it idles at 1100. Idle air makes no real difference. Someone said dialing air bleeds could actually help, but you do not run a Holley so I don't even know if you can do that.
That same tight lobe sep is great for scavenging and it actually brings down the torque curve and always makes more mid-range torque. Especially if good tuned headers are used. Circle track race cars use them for mid-range when coming out of the corners.
 

Joe473

Sr. Member
Joined
Jul 16, 2012
Messages
950
Everything on the engine is new except the RPM intake and the GT40P heads which were redone at the machine shop. I used new gaskets for the intake and took great care installing them. I only have 3 vacuum ports on the carb in the front and one on the rear of manifold going to the C4 trans. The big port on the back of the carb is plugged. I tried the 2 front small ports in the front for the vacuum advance while blocking the other. Then I blocked them both and used the manifold port for the C4 and still won't idle when warmed up. How would I find a Intake leak without removing the Manifold?
If you close the choke partially or all the way when warm does the engine idle faster and smoother? Try holding hand over top of carb to restrict airflow. If engine speed increases and smooths out suspect large vacuum leak

Did the shop doing the heads. Upgrade valvsprings and check seat pressure and clearances at full lift? Spring bond etc

Did you confirm pushrod length? Make sure there is proper lifter preload and not so much valves are hanging open? What rocker arms are you using?

Did you use end seals or silicone on intake to block end rails?

What does cam vendor say about base idle and base timing for cam and your compression ratio? If cam produces low off idle vacuum some may recommend mechanical advance only no vacuum advance. Vacuum advance works better with smaller cams.



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68ford

Bronco Guru
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Dec 26, 2004
Messages
2,710
Thumper cams are on a 107 lobe separation. The least aggressive one has more duration than my Isky. That has A LOT of overlap. I HIGHLY doubt there is anything wrong with his engine, that cam has a lot of overlap. I would suggest more lead timing. The overlap bleeds off so much compression that is likes more timing. Mine is 10.5 to 1 compression with iron heads and I run 20 degrees lead timing at idle. 36 degrees total.
 
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pennyduke

Sr. Member
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Jul 18, 2010
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377
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Thanks for the reply's.


Won't Idle: When I start it on a cold morning (50 degrees) it will idle high until I hit the gas then it comes down and sounds pretty good. Put it in gear and it wants to stall. I have to feather it to get it started down the road. It runs really good through the gears then wants to stall when I let off the gas to slow down for a light or stop sign. If I keep one foot on the brake and the other on the gas I can keep it running. The further I go the worse it gets.


I unplugged the Vacuum to the C4 and put a Vacuum gage in its place and only seem to get 2-4 hg I think. I do not have power brakes at this point. The needle was jumping around so much and the motor kept stalling so I'm not sure.

It will idle if I bump it up to just over 1K. But when I put it in gear it slams and stalls.

I will have to get back to you on the Cam Specs. I know it was the smallest Thumpr Cam they make. I will look today.

I used the stock converter in the C4 and have 3,50 gears according to the Marti Report.

Closing the choke plate don't seem to make a difference at Idle. I will check that again today when it gets warmed up.

I installed the heads myself. The shop only did the valve job and added the upgraded springs. I used the stock pushrods from the 1974 engine. I did not do any checking for coil bind because I don't know how. I did forget to mention that the Valve train is very noisy. no ticking but it's just louder than any of my other builds. I kinda think this is where we are going to find the problem. The more I think about it.


The Intake was sealed with a large bead of Black silicone. I'm going to check for leaks today if I can get it to idle cold.


I will get back to you on the Cam specs. Thanks Everyone
 

SHX669

Bronco Guru
Joined
Jan 9, 2009
Messages
1,997
Well - Edelbrock says the RPM manifold is designed for 1500 -6000 rpms -- soo - combined with your cam -- either crank up idle to at least 1000 rpm or pull the cam or replace the manifold with a streetable one or both.
 

Joe473

Sr. Member
Joined
Jul 16, 2012
Messages
950
Thanks for the reply's.


Won't Idle: When I start it on a cold morning (50 degrees) it will idle high until I hit the gas then it comes down and sounds pretty good. Put it in gear and it wants to stall. I have to feather it to get it started down the road. It runs really good through the gears then wants to stall when I let off the gas to slow down for a light or stop sign. If I keep one foot on the brake and the other on the gas I can keep it running. The further I go the worse it gets.


I unplugged the Vacuum to the C4 and put a Vacuum gage in its place and only seem to get 2-4 hg I think. I do not have power brakes at this point. The needle was jumping around so much and the motor kept stalling so I'm not sure.

It will idle if I bump it up to just over 1K. But when I put it in gear it slams and stalls.

I will have to get back to you on the Cam Specs. I know it was the smallest Thumpr Cam they make. I will look today.

I used the stock converter in the C4 and have 3,50 gears according to the Marti Report.

Closing the choke plate don't seem to make a difference at Idle. I will check that again today when it gets warmed up.

I installed the heads myself. The shop only did the valve job and added the upgraded springs. I used the stock pushrods from the 1974 engine. I did not do any checking for coil bind because I don't know how. I did forget to mention that the Valve train is very noisy. no ticking but it's just louder than any of my other builds. I kinda think this is where we are going to find the problem. The more I think about it.


The Intake was sealed with a large bead of Black silicone. I'm going to check for leaks today if I can get it to idle cold.


I will get back to you on the Cam specs. Thanks Everyone
It doesn't sound like a vacuum leak. If cam is as big as 68ford suggests a stock converter is too tight and i agree you will need to raise base idle and base timing. Looser converter will allow you to raise idle, not slam in gear, and use midrange power of the cam better. A cam like that also likes a lot of compression. What is your compression ratio? If you put gt40 heads on a stock lower end with dished pistons you may have lowered CR some and not utilizing the benefits of that cam design.

You need to check valvetrain out especially if it is noisy. Late heads have different rocker arms.

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pennyduke

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My Compression Ratio worked out to be 9.6:1. I will check on that too. I have it written down somewhere.
 

SHX669

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Jan 9, 2009
Messages
1,997
2-4 hg is pretty low a buddy of mine is running 6-7 in his full blown race car . So you might have a vacuum leak or an issue with the valve timing or not opening or closing all the way .
Like stated - have you checked how many #s of compression you have?
 
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pennyduke

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I used the stock GT40P rockers and found zero lash on each cylinder at TDC compression stroke, Then torqued them to 20 lbs before it hit one full turn. They all hit 20 ft lbs at 3/4 turn. When it warms up the top end gets noisy. It just sounds stressed. I thought it was maybe the Cam or the larger springs. I will try to post a video of it running today.
 

Joe473

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Joined
Jul 16, 2012
Messages
950
I used the stock GT40P rockers and found zero lash on each cylinder at TDC compression stroke, Then torqued them to 20 lbs before it hit one full turn. They all hit 20 ft lbs at 3/4 turn. When it warms up the top end gets noisy. It just sounds stressed. I thought it was maybe the Cam or the larger springs. I will try to post a video of it running today.
That sounds like pushrods are ok. The lift on the cam isnt extreme so I think you will be ok with valvetrain. That's a lot of duration. I doubt that cam will idle at 800 rpm in a 302.

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68ford

Bronco Guru
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Dec 26, 2004
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You cam has 1 degree tighter lobe sep than mine and more exhaust duration. The tight love sep causes much more overlap than if is was a 112 or 114 like normal. You're describing exactly what those cams do. Mines the same. It has to idle at a minimum of a 1000 and your cam has more overlap. I'm VERY sure you have nothing wrong and it's all the cam. If we had a really good carb guy play with our carbs it probably could be better but I don't that much. I hope you have very free flowing exhaust to utilize the scavenging from that cam.
 
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