302 Thumpr Cam Won't Idle

[pennyduke]

I just did some more checking at 1000 rpm and my Vacuum is at 15 hg. When I turn the dizzy either way it gets lower. I also noticed some oil starting to come out of my new blaster coil? The smell of gas from the exhaust is extreme. Can someone recommend a good Cam for the street and light trails. I'm ready to scrap this one. Thanks

 

[pennyduke]

While idle was at 1000 I put my hand over the carb and it didn't change much until I tried to seal it tight. then it wanted to die. I also sprayed Carb cleaner around the carb and entire intake and the idle never changed. When I sprayed it in the carb the idle went down.

 

[68ford]

https://www.hotrod.com/articles/camshaft-shootout-lobe-separation-angle-tested-explained/

Really good read that explains tight lobe separation and it's positive effects on power especially torque, how it actually lowers and broadens the torque curve, but also how it creates real low rpm vacuum issues.

 

[gr8scott]

Can someone recommend a good Cam for the street and light trails. I'm ready to scrap this one. Thanks

https://www.summitracing.com/parts/edl-2122

I can get mine to idle at 400 rpm, but I keep it at 650 rpm. Comp cams makes one with the same specs.
Sorry, don't know the part #.

 

[pennyduke]

I like that Cam. Looks like it has good reviews. I'm tired of smelling like gas and not being able to Idle. Thank you

 

[Broncobowsher]

Idling at peak vacuum is a sign you have too much advance at idle. The more it is retarded the better it will idle. I remember being a tooth off once and really retarded and it would idle so slow you would think it stalled then it would fire. Idled at something stupid like 200 RPM. Pretty amazing to watch.

Gas out the exhaust sounds like a carb issue. To small hydrocarbons the engine is either running with a lean misfire and the gas is simply not burning and being pumped out the exhaust. Usually a problem when someone tries to lean an engine out to pass emissions. The other is really rich. Pull the plugs and they are sooty black. The CO will be through the roof as well if you emissions test it. I've seen this a few times when a needle and seat get a bit of garbage (like a sliver of rubber fuel line) stuck holding them open. Watching the gas drip off the venturis at idle. But runs good under load, plenty of fuel.

I wouldn't be surprised if your cam problem is actually a carb problem, compounded with a little ignition problem.

 

[pennyduke]

Very interesting, I never understood the dizzy being off one tooth thing. Can you explain it to me and tell me how to check it?

 

[Master Chief]

The Comp link that you posted states, "best with 2000+ converter". Since you are running the stock converter, I bet this is your idle problem. I recommend calling ACC Performance and discuss your symptoms with them. Their number is 888-267-7464 M-F 8 AM – 5 PM CST. You may have to leave a message but believe me, they will return your call.

This is also a good explanation, https://www.onallcylinders.com/2016...e-setup-relates-torque-converter-stall-speed/

It is also worth noting that my 331 stroker made more power on the dyno with a 1404 (500 cfm) than a 1405 (600 cfm) Edelbrock carb and a 600 cfm Holley. You may find that you have too much carb for your engine.

 

[blubuckaroo]

I like that Cam. Looks like it has good reviews. I'm tired of smelling like gas and not being able to Idle. Thank you

You picked it.
Your video sounds just like I would expect according to the Comp Cam description.
That cam was designed to make a regular motor sound like a high performance motor.
http://www.compcams.com/Thumpr/index.html

 

[pcf_mark]

I think I am the guy who said "Don't Do It!". I have worked with two now they are garbage.

What I can tell you is you need to get deep into your carb and ignition tuning. I bet your exhaust stinks like it is pig rich?

Ignition - the cam will take a huge amount of spark down low and not knock. You need a ton of spark advance to get it to idle - 18-26 or more. We ran 22-26 plus the vacuum advance connected to manifold vacuum giving us over 30 degrees at idle. At idle - that is not a misprint. This gives you enough torque to make it idle and move out. When you get on it the vacuum drops and some advance goes away so it reduces chance to knock. We had some issues with the pulsing vacuum signal at idle causing the vac advance to move in and out a bit but it was the best way to get it to idle. This made the carb slow to come back to idle. This was a stick shift car. For an automatic this strategy will not work you need to bring the mech advance in earlier.

Carb - I do not know beans about Edelbrocks. Sorry. But if you have a vacuum enrichment circuit like a Holley power valve you need to change the valve. Your idle vacuum is maybe 6-9 inches of vacuum. At that vacuum a Holley stock power valve is pulsing open and shut at idle (making the exhaust stink). We went up in power valve sizing to a 10" vacuum and idle cleaned up nice. This was after several weekends of tryng everything else that made sense but the carb was so confused by the vacuum signal it was getting from the cam.

Sorry bud - those cams are all show and no go. Even once you get it tuned it is really hard to live with. Plus it makes mediocre power.

Post up your idle vacuum fully warmed up. Then again while driving down the road at your cruise speed. Those two vacuum points can help your tune.

Make the most of it by tuning but both of the engines I took those cam out of the owner was much much happier with the replacement.

 

[pennyduke]

Yep, It was you. Sorry I didn't listen. I'm weighing my options at this point. I read the TCI article above about stall speeds and it seems that is a big part of my problem. But the Converter I need would be $329 and it may not solve all of the problems. The Cam kit above is only $189 and looks like it would have very nice street manners. The Cam install would be a lot easier in my opinion. Hmmmm


pcf_mark, Did you look at the Stall speeds on any of those setups before the cams were removed?

 

[DirtDonk]

Also, what firing order is the cam? Not that this would help, but if it's different from what you're running that would certainly make things worse! However, from Mark's description it sounds like it's mostly the cam and not any discrepancy in firing order.
But I'm curious anyway since Comp did not see fit to post that up in the specifications for some reason. At least not that I saw.

The Edelbrock carb probably likes this type of cam even less than a Holley does. But different things can be tuned as well.
One pertinent aspect is the spring under the metering rods. The typical spring is stiff, because it's expecting a 16-22 vacuum reading. You would want to install their lowest rate spring I would think.
The result of the stiff spring and low vacuum signal is that the metering rods are always pushed up to the richest part of their range. Always enriched in other words.

But the carbs are very tunable however. Different metering rods, different jet sizes, different spring rates, and that kind of stuff.
So you may still be able to make it better, but as you're finding out, your engine is a system and all the parts have to be properly chosen to work together. The cam manufacturer's recommendations of which torque converter, which valve springs, which cylinder heads/compression ratios and all that stuff, is not a mere suggestion to get you to spend more money. They are practically gospel.
The only real exception to most of them are when the installer knows of slightly different components that might work even better. But very little will remain stock.
You certainly have a range of intake manifolds and carb sizes you can work with, but they would all be catering to a roughly 2000 to 5800 rpm power range dictated by the cam.
That cam might be cool in some applications, but only if built around it.

I think you're thinking in the right direction. Better to get a more appropriate cam than to keep trying to make the engine run better with the one you have now. You're throwing more money at it either way, but with a better goal by changing cams.

In my opinion anyway.

Paul

 

[68ford]

https://www.hotrod.com/articles/1110phr-testing-three-cams-on-a-347-ford/
There's a 347 in that article with a thumper cam and 2 other comp race cams. That engine also has big head and wide open exhaust being on a dyno. Which is needed to take advantage of scavenging the tight lobe separation and long exhaust event that takes place. it made very impressive HP and torque and proved to be the best all around. High peak torque at a lower RPM and broader torque range. They just have a really bad idle. I'm sure you are not running heads and exhaust to take advantage of that cam. And the idle will always suck. But my point is the cams aren't that bad if used in the right circumstances.

 

[tirewater]

You can save some money and get the Summit variant of that cam, the Sum-3600, or Sum-k3600 in kit form.

 

[broncosam]

I put a pretty aggressive cam in a 300 I6 engine I rebuilt years ago,it was in a 79 F150 4x4 truck I had. If I remember right it was a Comp Cam. I could not get it to idle without turning the idle speed up and then the engine would load up if I was crawling along in low gear. I kept playing with the timing and carburetor adjustment trying to get it right with no luck. I ended up calling Comp Cams and talked to a tech there and he told me to start over and gave me a starting point to where I should be with timing, I was way off. Anyhow after talking to him, I got it timed per his advice and with the carburetor adjusted to match,it idled good at about 600-700 RPM and the power and torque were unreal. In low gear with the transfer case in 4 low it felt like that truck could have pulled a house off it's foundation. So with that, if it were me I'd call the cam manufacturer and talk to one of their techs. Tell him your complete set-up ,I'll be willing to bet they can steer you in the right direction.

 

[pennyduke]

I really appreciate all the help. I have decided to make the Cam Change. I would like to stay with a comp cams version of the Edelbrock performer plus. I will call them today and see if I can get any credit for the Thumpr cam. Adding a spring kit and a torque converter may fix the problems, but I think changing the Cam will definitely solve my problems.

 

[68ford]

Pennyduke ask around to the cam manufactures before buying a cam that's the same as an Edelbrock. Those are old cams that have been around a long time. Old tech. They obviously work that's why they're still made, but pretty much any performance Ford forum you go on will say there's better newer cams out there. Maybe consider converting to hyd rollor? They make high bar hyd lifters now so you do not need the spider in the valley.

 

[pennyduke]

Good idea 68ford. I will check around and see what's out there. Thank you

 

[pcf_mark]

Comp's tech line is very good. I would call them with your engine, trans, gear, converter spec, weight and get their recommendation. They will nail it. You will pickup at lest 50 ft-lbs right off idle.

Thumpr are good on a dyno in a tight rpm window. Living with them day to day is just awful. They are garbage and meant only for people who want a dragster sound but have some life in the valvetrain.

 

[rmk57]

Comp Cams Xtreme energy XE 256H or the XE 262H are two I'd be looking at for your application.