Twin can 88 compression test, whats acceptable - Harley Davidson Forums: Harley Davidson Motorcycle Forum
 
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post #1 of 7 (permalink) Old 09-20-2019, 11:31 AM Thread Starter
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Twin can 88 compression test, whats acceptable

So, Ive got two FXDX bikes. One, a 99, the other an 04, carbed, and both have comparable mods.

The 99 has 37k miles, a Thunderheader, Big Sucker intake, stock pistons, and SE 204 cams, and made 86 lb ft and 79 hp on the dyno.

The 04 has 56k miles, a Supertrapp 2-1 with closed end cap and 20 plates installed, Big Sucker intake, stock pistons, Mikuni HSR42 carb (50 AC pump nozzle, AC pump set to engage immediately and full travel, 98 needle with clip in middle, 17.5 pilot, 160 main jet) and, according to previous owner, SE 203 cams, which are close to SE 204s, from what I can tell by the specs.

The issue is that the 04 has noticeably less torque than the 99, so Ive been investigating why. First, I was thinking perhaps cam timing was off, so I did a compression test to see if there was a big difference in readings between the front and rear cylinder on the 04, as Ive been told youd see around 30 psi difference IF the cam timing was off a tooth. I also did both bikes, so that Id have a basis for comparison.
Firstly, 160psi is perfect for the 88ci motor from what can tell, with these cams installed and OEM headgaskets and pistons.
04 has 140 psi in both holes. 99 has 155psi in both holes. I dropped about half an oil cap full of oil into both holes on the 04, and the compression reading went to 150 psi.

So, if 160/100= 1.6, and bike should make 79 ish hp to the wheel and 79/100= .79 and 160- 140= 20/1.6= 12.5% loss which would theoretically be 10hp if the correlation between hp and compression is linear.
IS 140 psi enough to make this bike a slug in comparison to my other bike with 155 psi? I also was wondering if the crank case was filling with oil due to sumping and that was slowing it down, but I took it for a rip and then promptly pulled the plug from the bottom of the crank case, and only 1-2 oz of oil came out, so its not that.

Im going to perform a leakdown test tonight to see if the valves are sealing properly next. Im just wondering, the head gaskets are clearly stock, so Im sure adjustable pushrods were used in the cam install, and if theres major leakdown from the valves, could it be that the rods were adjusted too long and are preventing the valves from seating fully? If the rods were adjusted too short, could they be keeping the valves from opening fully, thus giving a false lower compression reading and also hindering performance? If either of the previous mentioned issues were present, wouldnt the valve train be noisy and perhaps have grenaded by now?

Last edited by fxdls110; 09-20-2019 at 01:48 PM.
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post #2 of 7 (permalink) Old 09-20-2019, 01:12 PM
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Wonderful. Are pistons stock compression, how many miles...? Now go introduce yourself. Write a long dissertation just popping in here. This is forum of bikers who enjoy our passion for HD's. We get testy and helpful with one another. This not some internet ask and ye shall receive some instant gratification web site. Show a little respect for us here. Go along way to help solve your problems. .

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post #3 of 7 (permalink) Old 09-20-2019, 01:48 PM Thread Starter
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Copy that.
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post #4 of 7 (permalink) Old 09-20-2019, 11:05 PM Thread Starter
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So I put the thing in fifth gear. I got the front cylinder close to the compression stroke. I put 5 PSI into the system and then I gently Rock The Wheel back and forth until there was no hissing out of the intake or the exhaust. I then had April hold the foot brake on the rear wheel so it could not rotate inject it to 100 PSI and with the main gauge reading 100 the other gauge was reading 97.5 PSI with no hissing out of anything not even into the oil crankcase. The exact same is the case for the other cylinder. So I do not know where my power is going. I pulled the cam cover to check cam timing. With the rear cam sprocket dot on the line on the cam chest, the front cams dot is at 9 oclock. Thing is, theres no dot on the crank sprocket, so the ****er must be on upside down, which could mean the cams are lined up but the crank is not, so tomorrow i will pull the sprockets off and see.

Last edited by fxdls110; 09-21-2019 at 12:26 PM.
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post #5 of 7 (permalink) Old 09-21-2019, 07:21 AM
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Originally Posted by fxdls110 View Post
So I put the thing in fifth gear. I got the front cylinder close to the compression stroke. I put 5 PSI into the system and then I gently Rock The Wheel back and forth until there was no hissing out of the intake or the exhaust. I then had April hold the foot brake on the rear wheel so it could not rotate inject it to 100 PSI and with the main gauge reading 100 the other gauge was reading 97.5 PSI with no hissing out of anything not even into the oil crankcase. The exact same is the case for the other cylinder. So I do not know where my power is going. I pulled the cam cover to check cam timing. With the rear cam sprocket dot on the line on the cam chest, the front cams dot is at 3 oclock. Thing is, theres no dot on the crank sprocket, so the ****er must be on upside down, which could mean the cams are lined up but the crank is not, so tomorrow i will pull the sprockets off and see.
The pinion sprocket has a flat that aligns to a corresponding flat on the pinion shaft, only fits on one way. Same with the rear cam sprocket, keyed to the rear cam splines. The cam timing marks on the inside of the cam plate is the critical alignment. Those marks are on the plate side of the cams and can't be seen with the cams installed in the plate. The procedure is to transfer the marks to the other side of the chain sprockets with a marker so they can be aligned as the cams are pressed into the plate.

Disclaimer : Don't believe anything I say unless you are absolutely sure that I know what I'm talking about .
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post #6 of 7 (permalink) Old 09-21-2019, 01:30 PM Thread Starter
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I apologize, guys. I meant to say the front cam mark is at NINE oclock, with the rear cam sprocket mark on the line on the cam chest, and I edited it to reflect that. Just to be clear, regarding the absent timing mark on the pinion sprocket, what I meant was while its keyed and only fits in one position, if it was flipped so the timing mark is on the back side, it would be easy to have the crank off a tooth one way or the other. The mark on the pinion sprocket on the 04 very well could be under the washer. When I built a 99 for my girlfriend, the mark was on the tooth of the pinion sprocket, so I just expected the mark to be the same on this one, though Im aware they changed quite a few parts between 99 and 04. Guess I will go buy a new pinion sprocket bolt from HD, as youre supposed to replace em any time you remove em per the manual, and I will pull the bolt and washer and see if the timing mark is under the washer and confirm alignment.
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post #7 of 7 (permalink) Old 10-09-2019, 09:02 AM Thread Starter
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Well, Im going to take it to the dyno this week, so we will see if its just my butt thinking its low on power. Im probably going to do a 95" on it next month anyhow.
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