Using Carburetor Synchronizer Gauges To Improve Honda CB750 Cafe Racer Motorcycle Running | 27

[@10summoners]

Thanks for turning your frustration with this bike into our education! Nice work!

[@SPANNERRASH]

Thanks, I'm glad you find it useful. I can't wait to ride this bike :)

[@roum22]

Looks like you did a good job bench synchronising those carbs, they were pretty close straight of the bat..

[@SPANNERRASH]

Thanks. They weren't as far out as I expected, I thought the gauges might have moved more, but when adjusting I could clearly see the changes. I am just pleased that thre doesn't seem to be an issue with one of cylinders not running now.

[@rickmcconnell6520]

Sure sounds better! Can't wait to see her street ready. I still run my '73 with drilled out jets ,aluminum velocity stacks and drag pipes just like my brother-in-law set it up when he bought it new. Still scary fast!

[@SPANNERRASH]

Cheers. I think the rain and cold has slowed the pace down :( Interesting that you drilled the jets, I'm thinking of upping the size of mine in anticipation of Spring!

[@CarlAyers-x8h]

Built my 1971 750E to 836cc, No air box. Still ran 140mph. With 3/4 race cam. Idle at 900rpm. Open headers. Rigid frame. Kick only. No front break or Turn indicators. 50yrs riding Age.67 veteran 1Cav. I ride no more. Sold it..miss it. But I still have another running, 1969 E motor.. # 2396948.. no frame. :)

[@SPANNERRASH]

Sounds like a great build in the past. CBs really lend themselves to putting your own stamp on them. You should be able to pick up a titled frame for than '69 motor, maybe look for a rolling chassis, something with a blown motor. Take care.

[@neilsmovieshd]

The suction for the gauge is dependent on how much the restrictor valve is opened so you need to use one gauge as a reference gauge and then set the others on the one cylinder to get the same reading on all gauges of that cylinder before checking the others otherwise you won't get a correct reading. Also NEVER rev the engine much off tick over with these gauges otherwise you will ruin the gauges internals!

[@SPANNERRASH]

Thanks for the tips.

[@Floshen]

Great video. Those look like CV carbs, which run notoriously poorly without the airbox because they were designed specifically to be used with the airbox - the combination of the velocity stacks and the airbox itself means that the carbs can suck in air that is all at an equal pressure & speed and is all entering the carb in the same way due to the venturi effect generated by the velocity stacks. If you run stacks without the airbox, you're still ruining the venturi effect due to crosswind and pockets of higher/ lower pressure air. No real amount of rejetting or synching will get the bike running as well as stock. If you have a set of PD carbs, I would use those. I was able to successfully jet & tune my PD carbs for K&N pod filters + open 4:1 exhaust without too much trouble. The PD carbs work differently than the CV carbs and are much more forgiving when running stacks/ pods. The stock jetting is usually #35/#105 (pilot/main), I went up to #40/#120 and it runs like a sewing machine and moves like a scalded dog. I kept the needle in the stock (middle) position and my plugs are all looking healthy.

[@SPANNERRASH]

Thanks for the advice. I know that stock air filters are probably best, but don't look as good. Great information about using the PD carbs. I have a set on the shelf. I might try them if I have lots of issues once I can actually start riding the bike.

[@cabovermike]

Loving your work spanner rash,. i reckon a good grade fuel, optimum/v power or a good fuel additive , air filters and end can on and it should be spot on,. "hopefully",.. cheers,.

[@SPANNERRASH]

Thanks. I am sure it would be fine to ride, and good fuel is always a good option. I am thinking of increasing the size of the main jets before I get around to riding it. I suspect it will need a little more fuel when the throttle is towards fully open. If wasn't Winter I would ride it now, but while I have some time, I might as well get on with it.

[@1234567marks]

Hi Les, my CB has the later PD carbs, it would not run properly without the original air box or pod filters fitted, and in fact even with pod filters fitted it had issues, that’s because the carbs need a smoothed out airflow, that’s why in the original air box there are rubber velocity stacks, let’s face it, Honda wouldn’t have gone to the trouble of fitting them if they weren’t needed, now as I say that was with PD carbs, not the earlier ones that you have. The issue with most pod filters is that there’s nothing to direct/smooth out the airflow, so the air tumbles as it’s drawn into the carb upsetting the Venturi effect. When I fitted standard pods it ran reasonably well but wouldn’t hold a steady 70mph, it would surge, so I took the rubber velocity stacks out of the original air box and grafted them into the pods, it instantly fixed the problem and now it runs perfectly, you can see the rubber velocity stacks in this video, they protrude approximately one third of their length into the pods. So if you get poor running I’d suspect that👍 kzbin.info/www/bejne/mqvcoWZ3ZrmEeLssi=xwEIQCkfFid3jAju PS ignore the front mudguard, it was the original guard that I lashed on temporarily with zip ties to get me to the Dyno on a very rainy day!

[@SPANNERRASH]

Hi Mark. Great info. I have watched your video, looks like it is a great bike. The idea of the velocity stacks is great. I will see if I can find some and might try it. I have a set of PD cargb on the shelf, but I always seem to prefer the earlier ones. I am thinking about jetting for the future. From what i have seen on forums I think the 42 pilots should be right, but I have seen lots of different advice on mains. I iwll have to try the velocity stack idea first but I am thinking of getting a set of different sizes so I can do some plug chopping when I can start riding the bike.

[@1234567marks]

@@SPANNERRASH I put 42 pilots in mine, standards are too small once you have taken the standard air box/filter off 👍

[@SPANNERRASH]

Cheers

[@quercusdk]

Great video, helped me a lot. What's the tool you use at the end of the video and what are you measuring?

[@SPANNERRASH]

I think you mean IR Heat gun that I am using to measure the temperature of the exhaust pipes to see if any of the cylinders isn't running. It used to be that you would lick your finger and touch each one, if it didn't sizzle it wasn't running, using the gun doesn't give you burn't fingers :)

[@daveco1270]

Very nice. I have trouble getting the brass rods in place with the choke mechanism on mine. I can do it, but I've broken a couple of those tubes. Even with different sized tubes, it can be tricky. I'm curious if you have a ballast resistor in place with the 3 ohm Dyna coils. The stock coils on the CB750 were 5 ohm, which Dynatek makes...they're the black coils. (I put them on my bike when I added the Dyan electronic ignition.) I remember watching Hackaweek's CB750 series and he used the green Dyna coils but recommended a ballast resistor go with it. I'm not even sure what a ballast resistor is.

[@SPANNERRASH]

I did find using the extra tube from my old ballance kit, made it much easier to use the synchroniser. I was surprised the brass tubes didn't just work. I haven't come across any ballast resistors for the coils, I will have watch Dino's Hackaweek video to see if it is something I should have.

[@skydemon02]

Glad that did the trick.

[@SPANNERRASH]

I am a lot happier now, but I think I might have a bit more to do with the carbs. I can't imagine the main jets are big enough for the, big bore, pods and free flow exhaust.

[@philw4053]

Really enjoyed that. Do you have a link for the gauges you bought?

[@SPANNERRASH]

Thanks for the feedback. Here is a link to this type of synchroniser, ebay.us/g2cm5F , it is in the description as well.

[@philw4053]

​@@SPANNERRASHMany thanks (apologies - I missed the reference in the description).

[@SPANNERRASH]

No problem, glad to help.

[@aluheinzel]

You don't have cheap synchronous testers from China not so good

[@SPANNERRASH]

I have found them better than I expected. I usually swap the pipes between cylinders a couple of times to ensure that I can believe what they are telling me. But I do hear what you say.