Doc, the carb tuning on these bikes takes a bit of time from my limited experience on the CBR's. I've gotten 3 of my bikes running to verify that they were sound mechanically but only spent a little time playing with tuning. I rebuild the carb set that is on the conversion bike, but it hasn't run again since I tore it down.


I have a lot of experience with many different motorcycles and multi-carb set-ups, and would consider myself pretty competent with tuning. I have worked with tricky bikes (special builds that did not have stock fuel system) using O2 sniffers and am actually buying a system for the shop because I have so many bikes and ATV's that I am tuning that I figured it was time to just bite the bullet and get the proper tools to do this right (or at least verify that what I've done is right).


Cold start issues could be caused by a number of different issues. What I found when I was playing with the CBR's that I have is they are a bit cold blooded and they seem to starve from fuel at cold start up. This could be due to carbs not being perfect, and I still struggle some with your repaired diaphragm because the carbs really need to have consistent equal response. Even if it "looks" alright with 4 carbs they need to be tuned together properly.


The first thing to look at is the choke system. These carbs actually do not have a choke, but what is known as an "enricher". It's that little brass piston on the end of the choke cable. These can wear and stick. Even if the cable seems to be working the piston can be stuck in the bore and not actually working to feed fuel to the carbs. You can usually verify this in a number of ways...with the air box off and the choke engaged are you smelling fuel? That's the first test I usually do. You can also take the assembly apart and verify that it slides smoothly in the bore. Check the bore for scoring and the little piston. If not to bad a little emery can clean it up...but you can go too far so take it easy. There is a o-ring on the piston that needs to be good and may need to be replaced. You really need to verify the enricher is in good operating condition and is working properly.


The second area to look at is the pilot circuit. You have an aftermarket exhaust (one that I don't know much about BTW). If you have an aftermarket air filter and a modified air box, then you likely need larger pilot jets if you haven't put them in there yet. The pilot circuit adjustment is also critical and is very difficult to do on the CBR carb rack because the screws are impossible to get at.


Assuming that the carbs are in proper sync (this is also a critical factor), the proper way to adjust the pilot screws is with the engine running and hot (normal operating temp).


Each carb needs to be tuned independently in terms of pilot adjustment. I think we talked about this before and there is a bunch of total BS on the web about how to set the pilot circuit. The number of turns out shown in the manual is the recommended "starting" position after a carb rebuild or repair. You can't adjust this part of the circuit by just setting each carb to 2 turns out or some other number because the fuel mixture at idle and low end throttle (which is part of the cold starting part of this), is independent in each carb and when properly done each carb could be slightly different. This is probably because of machining variations or minor differences between each carb.


You start by adjusting the pilot screw on the first carb until you get the highest RPM from that carb. It's a little harder on 4 carbs because you need to listen closely to hear the change. Some will recommend turning the screw until the engine starts to "choke" of and want to die and then start opening the screw until the highest RPM is obtained in order to insure that the screw isn't already too far out to begin with.


You continue to do this until each carb is set at the optimum pilot circuit mixture. Generally a slight turn back on the screw is recommended (like an 1/8th turn) to insure that the screw doesn't open too far later on, but if the pilot jets are the proper size the screws should not be opened too far that they can change later. The screw position is also a method to determine if your jet size is to large or too small...and I think we discussed this previously.


As you tune each carb you may find that the RPM gets too high at some point and that can be turned down with the "Idle" adjustment screw. This is also a big mistake that people make when adjusting the pilot circuit. The pilot screws are the fuel mixture for idle and low end RPM, not for adjusting the idle.


Once you get each carb pilot fuel mixture adjust to optimum (again assuming that you have the proper pilot jet size in the carbs), then you adjust the idle to the recommended RPM and your good to go.


All of this assumes that the pilot circuit in the carbs has been properly cleaned in the first place...again this is an area that many make a critical error in thinking that just spraying carb cleaner and compressed air in there will clean it out...my experience is that even boiling the carbs does not insure that the pilot jets and passages are clean. The pilot jets are tiny. The holes must be probed with something that won't damage them but will help dislodge the crud that gets in there from sitting with gas in them or having sat for years with old fuel in them.


I use a strand of copper wire to get in there and not damage them. Then I look at them under a scope or a mag glass to see that they are open and clean.


I know this is a lot of info, but trust me...I have been caught here many times when I thought things were clean or adjusted right and they weren't. On many of my bikes and machines I come back and "tweak" them again because the perfect tune is tough to get to.


This stuff is not "Plug and Play" and a lot of people on the web that give advice are not competent in proper tuning and try to say that it is "Plug and Play". Trust me this is wrong. If you go to the carb manufacturer websites you will find directions very similar to what I just wrote...who do you believe? The guys that think the bike is running ok with the wrong settings or doing it the right way and knowing for sure it is set right.


There is a reason why dealer's charge a lot of money for tuning...it is more complicated than people realize.


The end is what I am doing. Having a pro style "sniffer" that measures the actual fuel/air mixture through the entire range of throttle to insure that all of the circuits are set to optimum and functioning properly.

 

Thanks so much Dennis, lots of great ideas. Once warmed up the bike is running pretty well, not absolutely perfect, but it will idle pretty well. BTW I did find a good diaphragm and replace the torn one. The jets were all cleaned in dip and cleared with fine wire, and passages were all blown out with compressed air. However I have not gone into the enrichers, and it does seem that the engine is fuel starved right at startup. As you say, the system is complex and you have to look at just one piece at a time. It took a little while to suss out that the enrichers could be starving the engine because overall it's running a bit rich. Checking the enrichment pistons is right at the top of the list in rechecking the setup.

I have also ordered fresh switch points for the fuel pump. It seems to be working OK, but I am not sure if the dressing I did on the burned points is keeping them from sticking all the time or if maybe they are getting stuck now and then. I'm kinda buried with work at the moment, but I'm hoping to get a day to get into all of this on the weekend.

 

This is an issue I had with all 3 of the CBR's when I got them running. The first one I could not get the fuel bowls to fill. Took forever to get fuel into the system. I had an extra pump but it only helped a little. I now have like 8 fuel pumps and hopefully will be able to get a couple of them working correctly.


That bike fired right up with a small squirt of starting fluid, but could not get it to run consistently on it's own until I stuck my hands over the intake tubes with the air box off...it just would not pull fuel into the carbs without help. My assumption on that one was it needed carbs done and enricher wasn't working because once I got fuel in there it kept running...and continued to run relatively decent.


The other one was similar but that one had a broken choke cable when I got it so I had to pull it manually. Similar on carb fill tho. It might be a characteristic of these bikes once the carbs are dry they take a long time to fill and come back to life...but sort of strange in my experience. The fuel pumps are sort of hokey in my opinion and I have looked at aftermarket pumps and I have some info on that somewhere if you find you want to go down that path. They have to be a certain pressure which is quite low and not a standard pump you can just go buy at the auto parts store. One thing that you might consider is your pump is not working. They will run on gravity feed if there is enough fuel head in the tank.


Fun huh? Ha! This stuff cracks me up, but we have to go through it and get it sorted so why not try and help each other.


Maybe we will get lucky and one of the guys on the forum will chime in on this...otherwise we are on our own.

 

Doc,


This is going to sound stupid, but I just got caught on this with the Raptor I was working on before my surgery. I cleaned the carbs and got the thing running and it ran pretty good and then started coughing and spitting so I tore the carbs down again and they were dirty again.


I was in a hurry to get the thing going because my back was going out and I knew I only had a day or two left and would be on my back and down. I had cleaned the tank, but didn't get it cleaned as good as I should have. I also realize there was no fuel filter on the bike...stupid. I got that straightened out and it ran pretty good but still needs some tweaking in the pilot circuit.


My point is don't leave anything out...maybe a plugged fuel filter...maybe dirt in the tank...who knows. None of us are perfect...just look what I did.

 

I do hear you and I am not too proud to go back in and redo things if it isn't working right. Already done that a whole lot on this project! Regarding the pump, one of the first things I did was replace the stock inline filter with one that has a clear (glass? plastic? not sure) housing, so I could see right away if my derust of the tank really got the rust out. So far so good on that, and the added benefit is that there is often a small bubble in there and you can see the pulse in the fluid when the pump pulses. It definitely pulses, but I have that same wonderment as you about whether the bowls have enough fuel to fill up on during a cold start.

I think I should have everything pretty well sorted from the tank to the float bowls once I get the new points. Plenty of clean fuel drains out of the petcock and I see plenty of fuel at the hose coming out of the pump. At the moment it's seeming like pulling the rack and checking the enrichment pistons is the first step. If that checks out I will get it started and warmed up one way or another and adjust the pilot screws and vacuum balance. If none of that sorts it I will recheck compression and valve adjustment.

 

Here's some interesting info.


I seem to remember reading somewhere, either during the development of the CBR or very early after introduction that they had a problem with fuel delivery.


If I remember correctly the bikes originally did not have a fuel pump. What they found was there was a fuel starvation problem at higher RPM (the float bowls were running dry). So they added the little fuel pump to correct this problem. I think it's why it's sort of weird and hokey in my experience with fuel pumps. I don't think it runs much in normal running of the bike because gravity feed is plenty...it kicks in and "assists" when needed.


Now, this recollection may not be totally correct and maybe someone will chime in with the correct story if I'm wrong (and it is possible I might be because my mind is not as good as it used to be), but if you think about it, it does make sense.

 

I may be on to a solution for the cold start problem. Bike starts perfectly with starting fluid, so it could simply be a fuel supply problem. This thread helped immensely:

https://cbrforum.com/forum/cbr-600f3...roblem-132611/

I pulled all the choke bypass valves and they were clean. While I had them apart I cleaned and buffed the rod tying them together and got a lot smoother choke action. Then I went into the fuel bowls again. Somehow I must not have been very observant the first time I blew air through all the passages. Sure enough, one choke fuel supply tube was completely clogged and two sounded like they might be partially clogged when blowing air thru. The process was shoot some cleaner in the tube, work it over with a single strand of wire pulled from from 20 ga. stranded wire, blow air, repeat until air blown thru each one sounded like the one that seemed clean. Did a couple of other minor tweaks to float levels and throttle valve alignment. Also got new switch points in fuel pump.

No time to test tonight, but hopefully I'll have a little more free time this week.

 

Doc,


I think you found the problem. Nice research by the way...now I have to go back and check mine...thanks...ha!

 

Just a brief update. Still not too much free time but I did get the fuel pump tested tonight. Initially I had tweaked the K&L points switch in installing it and knocked the pivot arms and spring apart. After much practice (!) I got that back together. Installed the switch and tried the pump. Just one click, hmm. Oh, duh, the center pivot lever thingie in the switch fits into a slot on the pump piston rod so it can move the switch points! A few moments of jimmying with the slotted mounting hole on the switch and it was all together as intended. Holy smokes that pump really cranks out the gas now, easily the 55ml in 5 seconds spec'd in the manual. For you guys in the US that's about a shot and a half

 

Got the carb rack back on today. Set full choke, and after a few seconds of cranking to prime the fuel line and bowls I got a nice smooth start and she ran great - good throttle response even when cold with the choke on. One more thing checked off the list.