Box Stock

Ignition timing

E

edman

New member
Posting this to get as much feedback as necessary/possible. Finished my clone build(for now)...shaved head .030, honda flattop piston, ported intake/exhaust, mod2 cam, bored stock carb on gas, stock size valves/rockers, hi flow filter/adapter, and header/no muffler.
Currently stock timing, wondering what is recommended by more experienced builders as far as timing?

also, when I pulled the stock head off this clone, I saw some detonation marks in the chamber. Higher grade fuel would help? Is timing also a possible cause of this too?

Thanks in advance, one and all.
 
I'll do my best to post a pic tomorrow. It looked like a series of small knurling/sharp-tipped punch-looking marks
 
Love your description, but it's hard for me to get a picture of what you seeing. With my 2 cycles, the detonation always looked like sandblasting around the edges of the piston.
Have you ever done a cc check of the combustion chamber?
Detonation is caused by excessive heat and/or pressure. From your description of your modifications, it's hard to say if you've got too much compression. It doesn't sound like it, not if you started with a stock engine. But that's just a guess. If the compression is high enough, and if you have the carb jetted too lean, that can cause detonation.
Another possibility is; the fuel you're running may be too low of an octane for the compression ratio of the engine. What octane of gas do you have now? Seeing that picture of the top of the piston would sure help.

Comments compliments criticisms and suggestions always welcome.
If the data does not support the theory, get a new theory. (Al Nunley)
 
Usually with very high compression the best results I've seen was retarding the timing with proper fuel to match the compression ratio. I once built a stock class GX390 Honda for tractor pulling. Had 17:1 compression ratio. Ran Sunco HCR Plus which it's made for 15:1 compression ratio and beyond. I started with a small 4 degree advance timing. Wouldn't do good. Went back home and took the motor out and flip the key around to 4 degree retard timing. Put it back on the tractor and went pulling. Won a bunch of pulls and it still puzzles me to this day that I had to retard the timing. First motor I've ever had to use a retarded timing key. Just thought I would share this. :) Each to it's own I guess.
 
Oh by the way Al. The reason why I did a 17:1 motor was to test your theory. Compression is the holy grail. But from what I seen the more compression you add the more the motor gets top end peaky. Runs good wide open and once it's starts to bog down it falls on it's face fast. Did I get more top end torque? Yes. Did I get more RPMs? No. Did I gain any bottom end torque? No. That was just my observation on my motor that I experienced through testing.
 
cubcadet70 said:
Oh by the way Al. The reason why I did a 17:1 motor was to test your theory. Compression is the holy grail. But from what I seen the more compression you add the more the motor gets top end peaky. Runs good wide open and once it's starts to bog down it falls on it's face fast. Did I get more top end torque? Yes. Did I get more RPMs? No. Did I gain any bottom end torque? No. That was just my observation on my motor that I experienced through testing.
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I have seen the same thing starting to happen at lower compression ratio.
It would be nice if an expert cleared the air as to why.

I have a theory of my own.
Beyond around 11:1 increases in compression start to give you a diminishing return on power increases.
You still continue to make more power, but the jump is not as big on gasoline as methanol. ( does heating of the charge have something to do with this? )
As compression increases so do pumping losses.
At part throttle and lower RPM I think these losses start to sap enough power to be noticeable.

I would like to see the pictures of the detonation marks.
I'm not sure what to expect.
I have seen some odd things on spark plugs from too much heat and timing.
But nothing you can finger as detonation until the engine is junk. ( maybe I just never heard it or could sense it in time )
 
cubcadet70 said:
Oh by the way Al. The reason why I did a 17:1 motor was to test your theory. Compression is the holy grail.
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you must have me confused with somebody else. I would never, ever, suggest running 17 to 1 compression ratio. I must have confused you about that statement, "compression is the Holy Grail."
Not to mention the fact that I find it hard to believe that you could get an engine to 17 to 1 compression ratio. Not saying it's impossible you understand, just improbable. What engine did you do this with, (full sweep CC's) and what was the final combustion chamber CC's?
It seems odd that you would come on here telling people you did something I told you to do, which I didn't, and then telling them how it didn't work.
Apparently you're not aware that there is a limit to the compression you can run with gas. That limit is controlled by several things, not the least of which is the octane of the fuel. The limit of available octane rating, with race gas, is right around 115. If they made race gas with higher octane ratings, you could run higher compression.
 
Somebody on the old bob 4 cycle form before it crashed named Al. But can't remember now. I must have had been confused with another person. He used to say compression is the holy grail. Yes I did a full sweep. CC the head and check the in the hole and checked the head gasket diameter and punched all the needed info in on a website. Brought out to be 17:1 compression ratio. It's on a GX390 engine not a GX200. I read on the Sunco website the HCR Plus fuel is made to handle compression ratios of 15:1 and beyond! But anyway. The engine never detnotate so I know it's good. I never said you told me to it. I just said I just tried out the compression is the holy grail theory from another guy. Apparently not you. But that's all ok. All I know my motor runs good and won pulls with it. No need to go farther.
 
Sounds like you may have run something thru the motor or possibly tip broke off of ring and bounced around. Were there the same marks on head?
 
No I do not. I ended up selling the motor and don't remember the specs. I'm getting the motor back soon to refresh it. I will check again and double check my calculation.
 
edman said:
I'll do my best to post a pic tomorrow. It looked like a series of small knurling/sharp-tipped punch-looking marks
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That sounds more like you dont have enough piston to head clearance and your piston is touching the valves
 
Didn't get pics taken yet; sorry.

Yes, there are similar marks on head. To the best of my knowledge, I was running 93 octane. This was all prior to my upgrading though; I bought this clone brand new in September, bone stock, and had maybe 30 minutes on it before I decided to make a little winter project out of it.

I noticed marks on stock head as well as piston, sure look like indications of detonation to me, but I'm no authority. Recommendations on fuel? Should I find octane booster, or try av-gas? I have jets on hand necessary to run methanol if that's a better option.
 
When I pulled stock piston out, I removed rings from it and inspected them. No breakage anywhere, and I set the valve lash prior to the initial starting of this motor in an attempt to avoid any potential contact/clearance issues. Again, this was on the stock head, with stock/dished piston.

I've since checked clearance/lash with the flattop piston and mod2 cam, no contact made.
 
cubcadet70 said:
Somebody on the old bob 4 cycle form before it crashed named Al. But can't remember now. I must have had been confused with another person. He used to say compression is the holy grail. Yes I did a full sweep. CC the head and check the in the hole and checked the head gasket diameter and punched all the needed info in on a website. Brought out to be 17:1 compression ratio. It's on a GX390 engine not a GX200. I read on the Sunco website the HCR Plus fuel is made to handle compression ratios of 15:1 and beyond! But anyway. The engine never detnotate so I know it's good. I never said you told me to it. I just said I just tried out the compression is the holy grail theory from another guy. Apparently not you. But that's all ok. All I know my motor runs good and won pulls with it. No need to go farther.
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Alvin Nunly is the same one from the old site who used to go by Al Nunly on there, he is the only AL that has been on bobs as far as i know, and the only one who always says "compression is the holy grail" lol. You didnt get him confused with someone else, no need to apologize for that. Sometimes Al reads things a little differently than what your actually saying, like when he thought he read that you said he told you to run 17:1 compression, when you actually said you built the engine with that compression ratio only to test his theory that "compression is the holy grail". Al is a good guy, he always trys to help other people out, he just sometimes doesnt read things the way they are meant to be read, and im sure hes not the only one who does that sometimes, i have too at times.

Im also interested in seeing how you were able to get 17:1 compression on any of these small engines, it seems like that would require a small amount of piston pop-up as well as a head that is milled down quite a bit, even then im not sure i can see how 17:1 is achievable, maybe 15.5:1 or somewhere around there could be done, but 17:1 is way too much compression for any build, especially with a small engine like these clones, whether it be a big block or small block. Heck, we never even built small block chevy or ford engines to that kind of compression lol. usually 13.5:1 or 14:1 was as high as we ever took them.
 
edman said:
Didn't get pics taken yet; sorry.

Yes, there are similar marks on head. To the best of my knowledge, I was running 93 octane. This was all prior to my upgrading though; I bought this clone brand new in September, bone stock, and had maybe 30 minutes on it before I decided to make a little winter project out of it.

I noticed marks on stock head as well as piston, sure look like indications of detonation to me, but I'm no authority. Recommendations on fuel? Should I find octane booster, or try av-gas? I have jets on hand necessary to run methanol if that's a better option.
Click to expand...

Methanol seems to be your best bet, but if you want to keep running fuel, try something like E-85 or at least some 110 race fuel, if your gonna use 93 i would definately find some octane booster, but i doubt those additives are going to help it much. I believe you would be better off just finding fuel that is already available in higher octane. 110 should be good
 
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