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Old 05-29-2014, 07:38 PM   #1
dukegnarley OP
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Joined: Sep 2011
Location: Bonney Lake, Wa
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do we have any engine gurus in here?

So... I need some help.

I just did the 790 big bore kit on the Dr650. I swapped the cam, had larger valves installed, put on a larger exhaust, and swapped the stock carb out for a TM40 pumper.

I can not get the thing to stop detonating under load. Here is what I have checked so far...

plugs are the correct ones and are gapped correctly.

Plug colors look fine.

I've tried running larger jets and they didn't help

The float level is set correctly, and I've tried moving it up just to see.

The alignment lines on the cam line up with the head like they are supposed to.

my compression tester says I'm getting 90 psi. (seems low?)


I am offering up some delicious beer to whoever can help me figure this thing out. I'm out of ideas, it looks like I've done everything correctly.
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Old 05-29-2014, 07:53 PM   #2
what broke now
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Far from a guru, but is your ignition too far advanced? 90psi sounds way too low for a fresh engine. Is the valve lash ok?
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Old 05-29-2014, 08:30 PM   #3
dukegnarley OP
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Quote:
Originally Posted by what broke now View Post
Far from a guru, but is your ignition too far advanced? 90psi sounds way too low for a fresh engine. Is the valve lash ok?
As far as I can tell the ignition timing it not adjustable. I just reran the compression test with the throttle open and it's about 115 psi.

The valve lash is within spec (0.006 intake 0.007 exh.)
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Old 05-29-2014, 08:34 PM   #4
dukegnarley OP
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Also, I'm getting a lot of popping on Decel.
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Old 05-29-2014, 08:42 PM   #5
AG200man
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Try running a colder heat range plug. What octane rating is your fuel?
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Old 05-29-2014, 09:00 PM   #6
dukegnarley OP
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Quote:
Originally Posted by AG200man View Post
Try running a colder heat range plug. What octane rating is your fuel?
The fuel is 92 octane, that was the first thing i tried

I'm not sure if there are colder plugs than the cr10Es I'm using, but I could try decreasing the gap.
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Old 05-29-2014, 09:02 PM   #7
Lake Cle Elum
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What is your compression ratio? You have added a lot of volume to the cylinder. There has to be a corresponding increase in volume with piston in TDC. This came be accomplished by Piston design, thicker head gasket, etc...I'd ask whoever made the kit and/or measure the volume.

If this was your issue, I would expect higher compression numbers though?
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Old 05-29-2014, 09:06 PM   #8
AG200man
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You're trying to effect the timing with the gap, I'm more thinking that with the mods you have done they have effected the dynamics of the head and the plug is over heating. You need to try a colder plug so it dissipates it to the head better. If it is the plug it is pretty severe and may need more than one range but you should notice a difference going colder by one number.

Something else I thought of - do you have access to anyone selling aviation fuel? Try it to see if the detonation stops?
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Old 05-29-2014, 09:12 PM   #9
dukegnarley OP
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Quote:
Originally Posted by AG200man View Post
You're trying to effect the timing with the gap, I'm more thinking that with the mods you have done they have effected the dynamics of the head and the plug is over heating. You need to try a colder plug so it dissipates it to the head better. If it is the plug it is pretty severe and may need more than one range but you should notice a difference going colder by one number.

Something else I thought of - do you have access to anyone selling aviation fuel? Try it to see if the detonation stops?
I really should not need to run av gas. if that's the case the bike is useless.
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Old 05-29-2014, 09:32 PM   #10
dukegnarley OP
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Someone somewhere else suggested that the cam was not timed correctly. I opened up the cam/valve cover and it was fine. What do you think the chances are that it was ground a couple of degrees off?

Jeff at Procycle said I'm the only guy who has had this problem.
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Old 05-29-2014, 09:39 PM   #11
AG200man
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Originally Posted by dukegnarley View Post
I really should not need to run av gas. if that's the case the bike is useless.
Yes it would, but it would also confirm some of your problems.
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Old 05-29-2014, 10:24 PM   #12
dukegnarley OP
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Quote:
Originally Posted by Lake Cle Elum View Post
What is your compression ratio? You have added a lot of volume to the cylinder. There has to be a corresponding increase in volume with piston in TDC. This came be accomplished by Piston design, thicker head gasket, etc...I'd ask whoever made the kit and/or measure the volume.

If this was your issue, I would expect higher compression numbers though?
the compression ratio is supposed to be around 9.5:1, the piston is dished.
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Old 05-30-2014, 04:40 AM   #13
eric n
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lean pop

you may be running lean. you've increased the volume of the combustion chamber without increasing your fuel mix. change the jet and create a rich condition then dial it back.

low compression may be because it is not worn in yet.

the hotter cams probably took the torque out of the lower rpm range. try riding higher rpm and less throttle position. (don't grunt it from the bottom rpm's )

find a auto repair shop with a gas analyzer and get a reading from it. otherwise its just guessing.

also try going loose on the valves. maybe a touch above the high clearance. "ticking valves are happy valves" but also slightly changes the valve timing. like loose on exhaust tight in intake changes where your torque versus acceleration is in rpm range.


an issue with changing the engine with high performance parts is you change the way its going to work. yes you may win a race but then it won't cooperate when trying to operate it normally.

and you could go deep into the rabbit hole

http://www.ecotrons.com/products/400...injection_kit/

with fuel injection that has mass air flow sensor with a oxygen sensor the programming will keep the fuel burn where it belongs. and can compensate for the other variables introduced by changing cam and bore.

eric n screwed with this post 05-30-2014 at 05:01 AM
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Old 05-30-2014, 05:43 AM   #14
HellsAlien
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+1 to getting an O2, A/F number off the pipe.

Another thing to check is squish clearance at the perimeter of the big piston.
I would think .010" min for something this big, maybe more. Can check that with solder through the sparkplug hole.

Another amazing consulting asset in your area is Dan Zlock, master engine builder at Zlock Racing down in Tacoma area. Best engine builder around. Might be worth a call, even though he didn't build your motor. Good luck.

I fought similar issues years ago building up a DR350.
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Old 05-30-2014, 05:53 AM   #15
dukegnarley OP
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Quote:
Originally Posted by eric n View Post
you may be running lean. you've increased the volume of the combustion chamber without increasing your fuel mix. change the jet and create a rich condition then dial it back.
Procycle recommends a 150 main jet, and I've gone all the way up to a 165. It didn't help, and my plugs were black.
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