Author Topic: High compression 73mm piston question. (Read 15138 times)

dragracer · « **on:** October 31, 2015, 11:08:55 am »

Just wondering if anyone has a high compression 73mm motor that they can still kick start. I mean over 12:1 compression. Would like to build one but want to know what I would be faced with as to kicking it.

gschuld · « **Reply #1 on:** October 31, 2015, 11:44:21 am »

No first hand experience, but I spent some time speaking with David Morgan, who owned the yellow dragster that Benton bought. He said that over the 30 years he raced that bike the motor has been rebuild into several combinations starting life as a stroked big bore motor down to a 12.5:1 compression 890 on methanol and Hilborns running 9.80s. His main starter was a crank drive outboard plug in, but has kicked it many times since he bought the bike in 1979 and raced it yearly since until a few years ago. He mentioned that his kickstart lever was shot peened and heat treated by RC Engineering. He specifically mentioned that he was constantly amazed that that shaft took the abuse for all that time. He said that he broke or bent a good number of Kickstart levers, but never hurt the shaft. He went on to say that his dad's motors never faired so well, breaking or damaging both levers and shafts. On all three bikes, I believe the kickstart was a secondary option rather than the main one.

If I were going to rely on a kickstart on a hi comp big bore, I would definitely consider having the shaft shot peened and heat treated, just like they do to steel connecting rods. Breaking out or bending the kickstert shaft would be an unfortunate reason to tear down a perfectly good motor.

George

joe p · « **Reply #2 on:** October 31, 2015, 11:57:09 am »

I ran 820, 836, 900, 1000,1080cc 12.5:1 with the head cut you can ask 754 how much he has the one I ran on the 1000 and 1080. I ran in the 10's but I was 250lbs. I used a RC shaft and never had a problem.

gschuld · « **Reply #3 on:** October 31, 2015, 12:06:21 pm »

For those who like visual aids and have not time seen this. RC Engineering catalog, circa 1978. Page 26.

George

MRieck · « **Reply #4 on:** October 31, 2015, 12:40:38 pm »

I have seen a bent kick starter shaft.

gschuld · « **Reply #5 on:** October 31, 2015, 01:36:06 pm »

On mine?

A high compression 1080 drag motor is certainly going to ask a lot from your Kickstart assembly and my comments were merely reporting the experience from one source. So take it for what it's worth. At least you are presumably going with an MSD ignition rather than the caveman like ARD magneto. That MSD should make things much higher easier on the spark and timing end all things considered.

George

bear · « **Reply #6 on:** October 31, 2015, 02:32:26 pm »

A kick starter is the primary starter for our Hy-Vo motors.

The rider needs to be able to start the bike if it stalls on track.

We also extend our shafts to accommodate the locker clutch assembly.

We have bent the kick starter a few times.
And the shaft occasionally, but not enough for it to be changed out before the 100 hour rebuild.

Our motors all run extremely high cylinder pressure and use the std mechanical advance with lighter springs.

Cheers,
Brian

cbr954 · « **Reply #7 on:** October 31, 2015, 04:06:59 pm »

The motor I built last winter with 72mm has 12:1 is kickable. When it is hot it is a harder to kick and wants to kick you back if you arent careful. Going to be building a outboard starter setup when I find a drive hub and block off plate...I dont want to take the chance in breaking or bending the shaft. But then again I never made it to the track this summer so I guess it cant break sitting in the shop lol

MRieck · « **Reply #8 on:** October 31, 2015, 05:44:13 pm »

Quote from: gschuld on October 31, 2015, 01:36:06 pm

On mine?

A high compression 1080 drag motor is certainly going to ask a lot from your Kickstart assembly and my comments were merely reporting the experience from one source. So take it for what it's worth. At least you are presumably going with an MSD ignition rather than the caveman like ARD magneto. That MSD should make things much higher easier on the spark and timing end all things considered.

George

Yes George. Funniest thing I have seen in while after mounting it it the lathe. Yikes!

gschuld · « **Reply #9 on:** October 31, 2015, 06:20:22 pm »

Hmm. I didn't recall that. I'm glad your were suitably amused...

Perhaps it's worth have a few kickstart shafts shot peened and heat treated to have on hand.

George

754 · « **Reply #10 on:** October 31, 2015, 07:03:15 pm »

I dont know about our motors, but Harley motor get lower cranking pressure from either increased duration or lift.. I suspect its duration.
I only ran 10.5 in my 836 and always kicked it..no started.. Once you prime it ( weber squirters).....it will light when you turn it over..

NalleyRacing · « **Reply #11 on:** October 31, 2015, 07:10:51 pm »

With Bils big cobra motor I could kick it first try in flip flops, I think there alot or factors but I think it has to do with theperson kicking the motor. You have to have the throttle opened to just the right spot and kick down, HARD!

dragracer · « **Reply #12 on:** October 31, 2015, 07:34:03 pm »

I've broke the small pawls in the kick start mechanism multiple times. I try to keep a few on hand. I've never bent the kick lever but then my engine isn't 12:1 either.

754 · « **Reply #13 on:** October 31, 2015, 07:49:45 pm »

Do you have to split cases to fix that ?

Have you ever roller started ?

seanbarney41 · « **Reply #14 on:** November 01, 2015, 03:41:30 am »

Quote from: 754 on October 31, 2015, 07:03:15 pm

I dont know about our motors, but Harley motor get lower cranking pressure from either increased duration or lift.. I suspect its duration.
I only ran 10.5 in my 836 and always kicked it..no started.. Once you prime it ( weber squirters).....it will light when you turn it over..

increased duration increases valve overlap time which bleeds off cranking compression, should have the same effect in any motor. I really have no experience on how much effect this would have on kickstarting an sohc4 motor.

TurboD · « **Reply #15 on:** November 01, 2015, 04:58:52 am »

This is a true test of one's kick starting ability.

dragracer · « **Reply #16 on:** November 01, 2015, 09:21:49 am »

Quote from: 754 on October 31, 2015, 07:49:45 pm

Do you have to split cases to fix that ?

Have you ever roller started ?

Yes, you have to pull the engine and split the cases, remove the kick starter gear, disassemble it to get to the one way ratchet piece. It usually starts with a slip or two of the lever when kicking. It'll make you think the lever is slipping on the shaft. Unfortunately, it'll be internal. I will likely send a few of those off for heat treatment before I build my next engine. Anything I build at this point must be self starting via kick or onboard starter. A reworked Paul Cashio super starter and 24volrs is a viable option. I just need to configure the starter gear set up to remove the weight on the end of the crank. Total loss system is what I plan.

dragracer · « **Reply #17 on:** November 03, 2015, 10:13:58 pm »

Quote from: cbr954 on October 31, 2015, 04:06:59 pm

The motor I built last winter with 72mm has 12:1 is kickable. When it is hot it is a harder to kick and wants to kick you back if you arent careful. Going to be building a outboard starter setup when I find a drive hub and block off plate...I dont want to take the chance in breaking or bending the shaft. But then again I never made it to the track this summer so I guess it cant break sitting in the shop lol

I've got a starter block off plate if you're interested. I don't have a starter nut though. In fact i think Bill Benton is taking a starter plate/nut off his new auto bike and installing an onboard starter. You might want to talk to him.

NalleyRacing · « **Reply #18 on:** November 04, 2015, 06:21:32 am »

Quote from: TurboD on November 01, 2015, 04:58:52 am

This is a true test of one's kick starting ability.

I sure wanna try

dragracer · « **Reply #19 on:** November 04, 2015, 07:01:12 pm »

Can someone post a picture of a 12.5-1 piston please. The bigger the bore the better, like 72-73mm in a k model dome. Or later F model if possible. Thanks

Old Scrambler · « **Reply #20 on:** November 04, 2015, 07:51:47 pm »

62mm 12.5:1 on standard rod & standard base-gasket.

dragracer · « **Reply #21 on:** November 04, 2015, 09:29:15 pm »

Quote from: Old Scrambler on November 04, 2015, 07:51:47 pm

62mm 12.5:1 on standard rod & standard base-gasket.

Thanks, just trying to figure out what direction I plan to go over the winter months. Decisions, decisions!!!!

cbr954 · « **Reply #22 on:** November 05, 2015, 05:04:12 am »

my 72mm mtc 12.5:1 pistons are not domed that high...just slightly more then the 10:1 ones.. I didn't get a picture before they went into the motors.

RP · « **Reply #23 on:** November 05, 2015, 05:50:51 am »

Increasing the bore size raises the CR even if the dome height does not get any higher. A lot of CR's stated by aftermarket companies are just guestimates on their big bore stuff.....we found that out when using a "whistler" on CBX prostock motors back in the day to measure CR.

Old Scrambler · « **Reply #24 on:** November 05, 2015, 07:55:44 am »

I changed rods to the longer CB900C units to push the crowns deeper into the F2 head. Did a little machining to the edges of the chamber to avoid detonation............

For big-bore ideas..............take a look at the Dynoman site...........Two crown profiles

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Author Topic: High compression 73mm piston question. (Read 15138 times)