Carb guru? Time to chip in (jet sizing question)

[big6fanatic]

I recetly found out #5 cylinder has a pretty low compression (100 PSI) I will look for the root cause behind this, but I'm tempted by a big bore kit if the problem is the cylinder/piston My head is already off (no my head but my engine head... ) I'm also looking to install individual air filters. I'm getting sick to tilt the engine to service the carbs
My questions mark is around the carb jet size. I have a Delkevic 6-2 exhaust and i'm close to see level (around 150ft). My carbs are VB64.

What would be a good jet sizes? I'm looking for something that will operate great from idle to red line with no flat spot and a resonnable fuel consumption.

Would a Dyna jet stage 1 work fine? I don't like the idea of drilling the slides however.

I'm really not a carb guru and my biggest concern is to have a hard time tuning the carbs properly. Too rich, too lean, hesitation, hard to start and ect. I want to ride all summer long not tunning carbs by trial-error mode. This is the main point that would stop me for going to a big bore/individual filters.

any advise are welcome. If some of you out there are using the same setup, please tell your story.

PM me if you want

regards

Martin

[daves79x]

Martin:

First of all - if you decide to bore, may as well go the 100cc as a stock overbore. Only additional cost is extra boring. Most big-bore kits out there are very good quality - pick the price you want to pay for one. For best performance from there, you really need individual filters and the Stage 3 kit, or serious stock rejetting. You have jet choices in the stage 3 as well as needle position. Most go with the smallest main supplied (only changeable jet in the VB64). Needle in leanest or next-to-leanest position. That would be close with stock-style pipe. If you want to rejet with stock parts, go from the stock 105 main to something in the high 120 range to 130. Shim the stock needle .020 inch. Set the pilot screws 2 1/2 turns out and that will be close also.

I've installed stage 3 kits with drilled and undrilled slides and couldn't tell the difference. Just keep in mind that you have VB64 carbs and you need a stage 3 kit (if you decide to go that way) for them - not the VB60.

If you get the jetting right, the bike will behave pretty well across the range and return 30 mpg. Jim Z. from Ohio has dyno'd several set-ups done the above way, with 6-1 exhaust, and sees 110 rwhp pretty consistently.

If you keep the stock airbox and pipes, I'd leave the main jet alone and shim the needle. Won't make as much HP as the above, but a quieter and more civil set-up.

Dave

[big6fanatic]

Ho thanks a lot Dave! That brings a bright light over a dark shadow I had. I'm gratefull to you. Since my first post I've been able to remove the cylinders from the engine in order to find out what was wrong with cylinder #5 with a low 100 PSI compression. I then found a good and not so good news. The good news is that the low compression on #5 doesn't seems to be related the to cylinder. As a matter of fact, I measured the Inside with a bore gauge and found within the manufacturer service limit tolerance.

Now the not so good news.

I found a huge groove in cylinder #3 (to my disbelieve). Stangely the compression was not so bad on #3 (150 PSI). The bore gage confirmed the groove is deep enough to require a bore oversize. Could not figure out at first why that happen. Then I had a stare at the piston assembly..... and there you go: Broken oil ring Still I was wondering why the broken oil ring. Then I recall that last winter i took off the cylinders from the engine and had a hard time realigning the pistons in the cylinders mainly on #3. I used steel shims to compress the ring and remember one of the shim went into the cylinder along with the piston. It was jam there and had to use long nose grippers to pull out the shim. Most likely I broke the oil ring at that moment.

It was a matter of time that this cylinder would go bad. Now the big bore kit is not really an option..... it's a must.

DSCF4289.JPG

[EMS]

the low compression on #5 doesn't seems to be related the to cylinder. As a matter of fact, I measured the Inside with a bore gauge and found within the manufacturer service limit tolerance.

Did you check the valves and valve seats on cylinder No 5? there must be a reason somewhere for the 100 PSI

[big6fanatic]

the low compression on #5 doesn't seems to be related the to cylinder. As a matter of fact, I measured the Inside with a bore gauge and found within the manufacturer service limit tolerance.

Did you check the valves and valve seats on cylinder No 5? there must be a reason somewhere for the 100 PSI

You are right. I did fill the intake and exhaust passages with solvent and found some leakage by the exhaust valves. I will have tout check that out. Thanks fort the hint

[bikeymikey748]

Martin, even without the groove in the bore, there are other issues that would contribute to your high oil consumption ( and not necessarily adversely affect your compression test results ). There's 'tattooing' on the walls from oxidation forming between where the piston stopped, and the bore. Usually, this occurs when the bike is stored for long periods and not prepared properly ( putting a bit of oil down the spark plug hole and cranking the motor through a few times). When the piston passes these regions, on the upstroke, oil is deposited in the depressions. As the piston descends, the rings cannot scrape the oil off the bore. As a consequence, consumption accelerates. Good news? The big bore kit will, easily, correct for this
As we've discussed, on my '82, I'm running pod filters + 1147cc. + 6-1. I have a Dynojet stage 3 installed ( 124 main, 35 pilot, needle in second position from the top ). I dropped the needle from the initial middle position, as it was running slightly rich. My plugs look pretty good now, and I have no fuelling issues from idle to redline
On a good day I'll be nudging 200km. before having to flip to reserve. Generally, I have to go to reserve at 180km., and that's with my accel. pumps adjusted on the low side of the scale, and trying to 'roll' through the throttle and keep them from activating too much.
Again, it seems odd the valve sealing concerns weren't indicated with your leak-down tests I know my #3 has,minor, issues with exhaust valve sealing. It wasn't apparent with a compression test, but was a few points off on a leak-down test. I just put my 'X' to sleep for the season. I should be checking my valve clearances this weekend, and am anxious to recheck them and see how they are doing 15,000km. later. I'll let you know what the results are if you'd like.

....and sees 110 rwhp pretty consistently.

Dave, I've never realized that our bikes could be coaxed to produce numbers like that with the mods you've mentioned

[daves79x]

To be clear, the HP gains noted are with the 100cc overbore, jet kit and pipe together.

Dave

[big6fanatic]

Martin, even without the groove in the bore, there are other issues that would contribute to your high oil consumption ( and not necessarily adversely affect your compression test results ). There's 'tattooing' on the walls from oxidation forming between where the piston stopped, and the bore. Usually, this occurs when the bike is stored for long periods and not prepared properly ( putting a bit of oil down the spark plug hole and cranking the motor through a few times). When the piston passes these regions, on the upstroke, oil is deposited in the depressions. As the piston descends, the rings cannot scrape the oil off the bore. As a consequence, consumption accelerates. Good news? The big bore kit will, easily, correct for this
As we've discussed, on my '82, I'm running pod filters + 1147cc. + 6-1. I have a Dynojet stage 3 installed ( 124 main, 35 pilot, needle in second position from the top ). I dropped the needle from the initial middle position, as it was running slightly rich. My plugs look pretty good now, and I have no fuelling issues from idle to redline
On a good day I'll be nudging 200km. before having to flip to reserve. Generally, I have to go to reserve at 180km., and that's with my accel. pumps adjusted on the low side of the scale, and trying to 'roll' through the throttle and keep them from activating too much.
Again, it seems odd the valve sealing concerns weren't indicated with your leak-down tests I know my #3 has,minor, issues with exhaust valve sealing. It wasn't apparent with a compression test, but was a few points off on a leak-down test. I just put my 'X' to sleep for the season. I should be checking my valve clearances this weekend, and am anxious to recheck them and see how they are doing 15,000km. later. I'll let you know what the results are if you'd like.

....and sees 110 rwhp pretty consistently.

Dave, I've never realized that our bikes could be coaxed to produce numbers like that with the mods you've mentioned

Guess who went by yesterday. YES Santa Clauss !!!


Bought this Kit and from the mouse clic to the doorbell in 5 days!. From Japan to Canada in 5 days. Amazing. Now time to get someone to bore my cylinders. Any ideas on piston to cylinder clearance?

As for my valves, plan to have the head and valves reworked (grind).

....Usually, this occurs when the bike is stored for long periods and not prepared properly.

Bikeymaikey, you hit the bull's eye, When I bought My X I had it stored for a couple of years (that wasn't the initial plan though)
Let us know how your valve clearance are doing
regards

[bikeymikey748]

It looks like an early Christmas for you, Martin
Congrats! As to boring, the best folks I know for the job are JPrecision:
http://jprecision.com
They aren't inexpensive. In fact, it'll cost more for the machining than what the piston kit cost. But....they do exceptional work. Tell them that you were referred by Guy Martin at MBP Ducati :
http://www.mbpducati.ca
Hopefully they'll (Julian at least) remember me and my CBX. They did great work on my SR500 too. Julian knows his stuff regarding clearances. He was also, happily, surprised at the quality of the big bore kit too if the boring work doesn't empty your pockets too much, they are the folks to do the head work too ( if you don't want to tackle that yourself).
Winter projects.....don't you just love them