Topic: My first complete build 72 CB750 - completed for now...

[Davez134]

Yes that's the dial gauge. My sensors on my pamco did not have screws on them, just kinda bent out. I looked at pictures of a newer one and see what you're talking about though. Would maybe have to unscrew, adjust, and make a tiny shim?


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[algophobe]

I finally had a little time to mess with the bike this weekend. two main issues i'm finding. to recap, i'm running the PAMCO ignition, with Keihin CR31 special carbs. two issues i'm running into now after having figured out there was a loose wiring causing coil 2/3 to not fire.

1. i'm using the strobe light to do dynamic timing at this point only at idle after warming up the engine. according to convention, one is supposed to use cylinder 1 or 4 to do the timing. I timed the ignition to #1 with the timing light clamped around plug wire one. then double checked with the clamp around #4 and it seems to be off or positional. I also compared to #2, which seems only slight off but acceptable, but the strobe light does not fire as regularly as #1. when checked with #3 it is even worse. at this point more or less both coils and pick ups on the ignition unit is working. so what's causing the misfires? it can't be the timing gun itself as #1 consistently produced good results and #3 consistently bad. but other wise the engine sounds like it runs fine.

2. with the Keihin CR specials, I manage to solve an issue with the engine idling better using the "wonderful fully detailed" manual that comes with the carbs (sarcasm). previously I noticed that the engine idles fine, but cuts out when giving anymore throttle than 1/8 turn. the air screw was too far out. I closed all 4 screws down and back it off 1/4 turn at a time. and seems like 1/2 turn was enough. she now revs smoothly. it seems when Dime City put it together, they set the screw 1.5 turns out (running it too lean.. am I right in this thought process?). but now, I'd like to balance the carbs as soon as I figure the timing issue... where the hell is one supposed to attach the vacuum gauge? I've search everywhere and no answer except making a hole in the rubber adapter going into the header. i'm assuming adjustments are made with the top cap off and the screws on the link lever. only thing I can think of is the small air leak jet underneath the link lever (see pic).

[Godffery]

 The CR's have NO vacuum ports.  =/
I have installed aftermarket ports in the intake boots and that work pretty well, but you need to make sure they are all installed exactly the same on each boot and with a nice clean drill hole.

 Here is a example (not my work)
 http://draftcycleworks.blogspot.com/2015/04/installing-vacuum-ports-for.html

[Terry in Australia]

CR's are a pain in the arse to set up, I was always going to tap the screw-in manifolds for vacuum ports, but I never really felt like mine were out of synch, so didn't bother. You could always take them off and bench synch them using a piece of wire for a gauge, but it won't be as accurate as a vacuum gauge.

With your timing light, I've had exactly the same issues with my old Dyna III ignition, looking at the erratic light on 2 & 3 you'd think it was missing, even though the engine was running well. The advice I got from members here is to ensure that the lights pickup is pointing in the right direction, apparently there's an arrow on the pickup to tell you which way to connect it to your plug leads? I haven't tried it since, so I don't know if this will make any difference. Good luck!

[algophobe]

thanks for the info guys! happy new year! to think how much money one puts into the CR's they and just drill a hole into the carbs themselves for you. probably because its modular but why not. I was beginning to think I was crazy not being able to find the damn ports. I may just tap directly into the carbs with threads that matches the carb sync system (if i'm ballsy enough)... have to do a little thinking.

as for the clamps on the strobe light, yes I did orient the arrow correctly. but I lateral realized that the knob on the timing gun was set to some crazy advance. i'll have to be more careful and retry the timing tomorrow.

[Godffery]

 I have found that when timing a modified motor like yours, I get the best results about 10 degrees advanced.

[algophobe]

I drilled some holes into the aluminum rear cowl and is regretting it. any suggestions on fixing 1 cm to 2 cm holes in aluminum?

You can either weld them up, or if you can't weld aluminum, you can use Aluminum Bronze fill wire and "braze" them closed. Using a propane (preferably MAPP) gas torch, you can heat the cowl up enough that the wire will stick. A few dabs, cool, then grind flush and smooth.

possibly an opportunity to learn TIG

A had a little time while waiting for some part. Decided to repair yhe holes i drilled into the rear cowl. Thanks to calj's suggestion of brazing with aluminum rod. Not bad for a couple of hours of work!

[algophobe]

so after a little bit of frustration and waiting for part for the issues of the timing and carb balance, I took a hiatus from that issue and worked on the front end for a little bit. bought some new parts for the front brakes, which I was not able to extract the frozen piston. I derusted and did a zinc recoat using the Eastwood kit and buff them to near new. I used a single SS brake line from the master cylinder to caliper. I initially was not going to run front fenders as the ones that came with the bike was chopped for some reason of the front 1/3. but I wanted to have a safe way to keep the lines off the tires, no I decided to chop what ever is remaining of the front fender. new horn from Joker machine.

[algophobe]

then fab a little bracket for the idle screw extension for the CR carbs. something strangely satisfying about make simple little tid bits like this. which brought me back to the frustrating issue at hand... which much advice is needed from you all.

I took davez134's advice and check the spark advancer shaft to see if it's true with a dial gauge. its only about 0.3 mm off.

so to recap. cylinder 1/4 runs great when using the timing gun. but when used to evaluate the 2/3 cylinders off the spark plugs, the firing is very erratic. I fired her up today, and the 2/3 pipes are barely warm. in fact the timing gun doesn't even pick up anything. so I decided to swap the coils and same thing happens. therefore it can't be the coils nor spark plug wires. I took out the spark plugs to check for spark. the spark plugs from 1/4 sparks well. and if I took these spark plugs and hook it to the 2/3 coils, it sparks well too. therefore it couldn't be the magnetic pick ups nor wiring leading up to the coils. but when I took the spark plugs from 2/3 and hook it to 1/4. they have a trouble firing. THEEEENNN!!! I notice a spot of what appears to be oil on the ground.. Thinking "Oh great another issue I have to deal with, a leak from the case". but all engine mating surfaces are clean as can be. apparently it was coming from the seam between the exhaust down pipes and the mid pipe coming from cylinder 2.

it smelled more like gas mixed with exhaust carbon and not oil specifically. and after some air dry time and minor cleaning.. the spark plugs taken from 2/3 fires well. I put a camera down into the head and 2/3 looks clean (in fact it barely looks like its been working). 1/4 looks like it's been working with minor carbon build up. I decided just for the hell of it to do a compression test and all cylinder produce about 140 to 145 psi.

now i'm all confused. HEEELLLPP!!! is it that the 2/3 is running too rich and fowling the parks? I set the air screw the same amounts of turns. I remember reading somewhere that the jets for 2/3 usually is a little bigger than 1/4 because they tend to run hotter being in the center. could that be it? that's why then ran for a short period then stopped working? any input would be greatly appreciated.

[Godffery]

 Those budget pod filters are going to mess with your air / fuel mix. Your best to go with K&N RC-1820 before even trying to tune it.

[algophobe]

Those budget pod filters are going to mess with your air / fuel mix. Your best to go with K&N RC-1820 before even trying to tune it.

hmm. you really think that's the base of my problem? why would it only affect cylinder 2/3 and not 1 or 4? but it would explain why it ran initially then gradually became erratic and then stops firing all together.

[calj737]

Those budget pod filters are going to mess with your air / fuel mix. Your best to go with K&N RC-1820 before even trying to tune it.

hmm. you really think that's the base of my problem? why would it only affect cylinder 2/3 and not 1 or 4? but it would explain why it ran initially then gradually became erratic and then stops firing all together.

Its not your current problem as to why 2/3 aren't firing, but as Jeff points out, they will hinder you long term in getting your bike running well.

[Godffery]

Those budget pod filters are going to mess with your air / fuel mix. Your best to go with K&N RC-1820 before even trying to tune it.

hmm. you really think that's the base of my problem? why would it only affect cylinder 2/3 and not 1 or 4? but it would explain why it ran initially then gradually became erratic and then stops firing all together.

Its not your current problem as to why 2/3 aren't firing, but as Jeff points out, they will hinder you long term in getting your bike running well.

Yes, it will help to start with a good base line by eliminating all known week links right down to the basics.

[algophobe]

Those budget pod filters are going to mess with your air / fuel mix. Your best to go with K&N RC-1820 before even trying to tune it.

hmm. you really think that's the base of my problem? why would it only affect cylinder 2/3 and not 1 or 4? but it would explain why it ran initially then gradually became erratic and then stops firing all together.

Its not your current problem as to why 2/3 aren't firing, but as Jeff points out, they will hinder you long term in getting your bike running well.

Yes, it will help to start with a good base line by eliminating all known week links right down to the basics.

ordered!  ... I guess you get what you pay for. I took this opportunity to take down the carbs and boots as well and try the mod as suggested earlier about putting vacuum ports into the boots to hopefully help with the carb balancing once I figure what the hell is wrong with my 2/3 cylinders.

[algophobe]

so I happened to run into an old friend at work and he asked about the bike. long story short, I told him the current situation. he recommended checking the valve clearance. I may come loose after a few times running the motor, especially when new valves and guides were placed. so I check first thing when I got home, and lo and behold. they were waaaay off. in fact all 8 valves. it may not explain every thing... so I proceed to adjust the valve clearance back to 0.002 and 0.003 " for intake and exhaust.

my question that with the compression test I ran yesterday producing 140-150 psi on all four cylinders. a loose tappet wouldn't affect compression.. is my thinking correct? because loose tappet would not open the valves wide enough to produce efficient intake and evacuation of ignited fuel/air mixture?

[Terry in Australia]

No mate, a loose valve clearance won't affect a compression reading, only if you have no clearances, as is sometimes the case with valve recession etc where the clearances are so tight that (especially when hot) valves can't entirely seat. Cheers, Terry.

[algophobe]

Took a needed break from the cb750 build and worked on my running 05 Kawasaki 636.  a little more brainless work on the known vs. the unknown of a custom bike. a while back I had install the Dyno Jet quickshifter. I committed myself to the "pull" sensor type and like to shift GP style. some clearance issues required that I raise the lever up.. which makes for a tired shin after a few hours of riding. started work on fabricating an extension tab for the lever. works pretty well. except I have to make another one to extender it a little more for more anatomically correct forefoot positioning. I mocked it off using an older crashed Sato rear set.

[algophobe]

so I decided to get new updated 5 ohm coils for the ignition system. I was concerned maybe the old type of screw connectors from the coils to spark plug wires were weak or loose. the engine fired right up. ignition was timed to +10, all cylinders firing consistently. and timing gun picked up all four wires. check full advance at 2500 rpm, it seemed off. but I thought I had solved the problem... went back the next day to see if I advance it to 15 deg @ 1000k and recheck 2500k full advance. and guess what I find. cylinder 2 was not firing again! I'm at my wit's end. I'm about to pull my hair out. from an electric standpoint the system runs fine, but fails after some time running. could the problem be coming from below? I'm out of ideas. do I need to pull the motor and take it apart?

[calj737]

Did you install new condensers? Have you checked the resistance of your plug caps? Your thread is pretty much all over the place, so pardon me for re-asking these questions.

You say #2 isn't firing, is there no spark at all, or no combustion? Is the pipe at all hot? There will be some heat from the spark going off even if the fuel isn't being combusted.

[algophobe]

Did you install new condensers? Have you checked the resistance of your plug caps? Your thread is pretty much all over the place, so pardon me for re-asking these questions.

You say #2 isn't firing, is there no spark at all, or no combustion? Is the pipe at all hot? There will be some heat from the spark going off even if the fuel isn't being combusted.

no worries. I took a little hiatus from this issue due to frustration. I'm running PAMCO ignition so no condenser. new plug caps, new plug wires.. so I don't think its a plug cap resistance issue.  2 fires when the new coils were installed. I checked for spark before placing back the spark plugs into the engine. but after running for 10-15 minutes to adjust advancement. combustion does occur initially and pipes get hot. but the next day, 2 won't ignite, the pipe is barely warm compared to the other 3. I swap the wires for 2 and 3 and again 2 doesn't produce combustion.

[calj737]

Fuel issue on #2

[algophobe]

Fuel issue on #2

so why would it combust initially but not the next day? any chance that there's something wrong from the piston and rings down that would cause it as well? btw the carbs are new as well. CR specials.

[Godffery]

 Any chance that plug is fouling out?  Have you tried swapping it out with one from another cylinder after it happens?
If you do that, and it fires back up in #2, then fails again after a while, I would consider a possible fuel issue. (If I recall correctly, you had some question about that in the recent past?)
 Also; have you done a compression test on all 4 cylinders?

[algophobe]

Any chance that plug is fouling out?  Have you tried swapping it out with one from another cylinder after it happens?
If you do that, and it fires back up in #2, then fails again after a while, I would consider a possible fuel issue. (If I recall correctly, you had some question about that in the recent past?)
 Also; have you done a compression test on all 4 cylinders?

you are correct sir! yes compression test were repeated on 2 separate occasions, consistent across the board. my initial suspicion was it fouling out. but are we talking about fouling from excess fuel or oil leak. I thought maybe there was an oil leak from the rings. but would you see a drop in compression? i'll have to try swapping the spark plug next chance I get.

also you refer to fuel issue, are you thinking the actual gas itself or the quantity coming from that particular carb?

on a side note, this may show my lack of knowledge but could using the wrong oil foul a spark plug sooner? I've traditionally used mobil  automotive oil for years on all my bikes without issues, as long as it is compatible with the wet clutch.

[Godffery]

 Yes fouling from excess fuel or oil.
If the oil was getting in from the rings; That would likely effect the compression (unless it was swimming on oil) But if it's coming from a valve guide, that would not effect the compression.
If it is from fuel; Then I would not say the fuel itself, but an excess of fuel that is coming from that #2 Carb. (possibly a float needle not seating properly?)