Bob Herders Thompson - help needed...

[chads]

I just finished up a Bob Herder's Thompson engine. I tried to run it but it only popped a couple times. The compression is very low and I changed out the CI piston ring with a rubber O-ring- Same thing

The valves seem to be seating fine. Has anyone ever seen this before on any other engines?

BTW, here is a picture of the engine. Chads

 

[krv3000]

HI you say the valves are OK ? have you dun a leek test tack of the Head and leave the plug in and put sum paraffin in the head and see if it leeks threw

 

[stevehuckss396]

Cam timing can effect the compression so make sure that is properly set.

 

[cfellows]

If your compression is still low, it could be the sparkplug seal or the head gasket. But, I would tend to suspect the valves first as they are usually the culprit.

Chuck

 

[maury]

What I do to check the valves is to assemble them to the head with the springs, making sure everything is dry. Then, squirt some wd-40 into the ports behind the valves, wait a few minutes, then check the front side of the valves. If they are wet, they are leaking.

maury
www.lonestarengineworks.com

 

[chads]

UPDATE..

I relapped the intake valve and now have good(better) compression. I still could not get it to fire.

However, I tried something different. Even though it is setup with points, I put my buzz coil box on it and it fires and runs for a few seconds. I realize the timing is way off because it is not set up for a buzz coil but it does fire.

Can anyone give advise as to maybe the spark is too weak ???

What type of coil do you guys suggest I run with this type of engine.

Thanks,
Chads

 

[JorgensenSteam]

As I understand it, you need to use a 600 or higher grit grinding compound such as loctite Clover on the valves.
The coarse stuff they call valve grinding compound that is readily available in the auto stores is way too coarse for this size valve.

Pat J

 

[Brian Rupnow]

PatJ has it right. The commercial valve grinding paste available for automobile valves is far too course for these small engines. I have fought this valve battle a couple of times now, and found that the "Final Solution" was to first make a special valve seat cutting tool, and then to use progressively finer compound, starting with 300, then 400, then 600 to get enough compression for these small engines to work well.000Brian

 

[chads]

The engine fires but cannot sustain running. Do you guys suspect this issue as a compression problem still? I'm out of ideas.
Thanks for the help...
Don
(Chads)

 

[cfellows]

Insufficient compression could be the problem. It might be firing, but if it doesn't have enough compression, it won't have enough power to keep running. When you spin the flywheel by hand, it should bounce back pretty good if your compression is sufficient.

If the fuel mixture was a problem, it probably wouldn't fire at all. I see you have a transparent fuel line, can you see that the fuel is getting to the carburetor with no bubbles in the line? Also, are you reasonable sure that the intake valve is letting fuel in? It could be that your intake valve spring is just a tad to strong.

You might also try just manually running it in a bit but hooking it up to an electric motor, your lathe, or something not too fast to spin it over for 10 or 15 minutes at a time to see if that improves the compression. Just be sure you oil it pretty good so you don't score the cylinder or bearings. Also, disconnect the spark and the fuel line.

Just some thoughts...
Chuck

 

[myrickman]

Recheck the timing carefully and spend some time making sure it fires close to TDC? On larger engines, when the spark is late they run as you indicated. With all the small parts on the ignition, something may be changing the timing when it is running.... It does sound like you have the compression at least at a useable level...

 

[chads]

OK Guys, I got her running

I lapped the valves with toothpaste over and over and the compression increased where it had the bounce. After some adjustments to the timing and the touchy carb, here is the result...



[ame]http://www.youtube.com/watch?v=r-tBIqpZ0C4[/ame]

 

[dalem9]

Very Nice ,you should be proud. Dale

 

[cfellows]

Sounds great, Chads. What kind of ignition coil are you using?

Chuck

 

[chads]

I am using a 12V automotive coil. The points I used were from a big old relay that I tore apart.

...and thanks for the kind words

Don
(Chads)

 

[kuhncw]

Congratulations on getting the Thompson debugged and running. Sounds good.

Regards,

Chuck

 

[ShedBoy]

That sounds nice and crisp. When I google this engine it shows a picture of one with a semi enclosed crankcase. Really nice engine, well done.

Brock

 

[chads]

I just have the crank cover off to troubleshoot. I ran it again tonight and it really is starting to pick up some rpm's. For some reason it will get an intermittant air bubble in the fuel line which will either cause it to stall out or run erratic for a moment.

Don
(Chads)