Having to hold starter for longer than i should...

[crozell]

Good morning Team. 79 with velocity stacks, rebuilt the carbs (multiple times), once the bike is running it is fine other than a slight hesitation from 2k-3k (raised the needle one slot which helped but still has slight bog).

Issue I am having is starting it cold, seems like I have to hold the starter for about 5-10 seconds for it to finally "catch" and run. Bike will pop and try to start while holding starter but requires constant starter to take off. When it does start running it dies shortly after and I have to repeat the process 3-4 times before it finally wants to run on its own. Once it is running it really likes to warm up before being ridden. I thought it had something to do with the vacuum petcocks getting fuel down into the carbs but I see guys tap the starter and it fires right up...

What do yalll suggest I check?

Thank you for all your help!

[DiverRay]

Start with the cold start jet in the bottom of the float bowel.
The 2-3K hesitation seems to be too small or wrong pilot jet.
If you have changed jets, did you use only Mikuni jets? Aftermarket ones as in the K&L kits do NOT work, as the size of the jet does not correspond with the Mikuni size guide.
Go to the carb rebuild guide and make sure you have all passages cleaned.
Now, just to throw a monkey wrench into things, have you checked the pickup coil wires? Starting and low RPM is when they seem to show up on the '78 to '80 bikes.

[Radioguylogs]

Are you using the choke (enricher)?

There are choke (enricher) passages built into the bottom of the float bowl that are the fuel supply for the choke. These passages get clogged. Have you sprayed choke cleaner through these passages to be sure they are open?

[crozell]

Thank you. Any way you could post a picture of what passage you’re talking about- choke enricher passage.

[DiverRay]

Here are a few pictures. Old, dirty carbs with a plugged passage. Notice that the carb cleaner is NOT going from the jet in the bottom of the bowl to the passage into the enrichment circuit.

[DEEBS11]

More thoughts: It can also be a voltage/current drop during starting. Some have described the bike as finally starting "just as they let go of the start button- it fires". Battery might have voltage but not enough current. Or possible bad connections.

Another thought is that you mentioned velocity stacks. You must have a larger pilot jet such as #45 and it must be genuine Mikuni.

[LAB3]

I'm with Deebs on the voltage drop. If tapping on the starter makes something different happen it's probably a sign of the starter needing attention.

[crozell]

Update: finally had time to tear into carb, 2 out of 4 choke (enricher) passages were fine, other two completely clogged. I have done two rounds in the ultra sonic cleaner and still clogged. Any tips on how to clean those? Confirmed that I replaced pilot jets are mikuni 45’s when I rebuilt it last, they are brand new. Anything else I should look at while I have the carbs off?

[cajun31]

A good soak in a gallon can of Berryman's will do the trick. Berryman Chem-Dip Carburetor and Parts Cleaner 96oz (autozone.com)

[DiverRay]

What do you use in the cleaner? I've found that REAL pine sol works well on carbs, without causing problems. A fine wire or long needle may help break up the crud in the passage.

[LAB3]

I've had varying success with Pine-Sol. One method I came across uses a 50/50 mix of Pine-Sol and water in a pot brought up to a boil. Drop the carbs in, let it sit in there for 1/2 hour then remove the heat and let them soak overnight.

I've also used Barrymens carb dip as well, it does a better job overall. The question becomes do you want to spend $40 and have it sit around until you need it again or if you decide to get rid of it how do you properly dispose of it?

[jetmechmarty]

More thoughts: It can also be a voltage/current drop during starting. Some have described the bike as finally starting "just as they let go of the start button- it fires". Battery might have voltage but not enough current. Or possible bad connections.

Another thought is that you mentioned velocity stacks. You must have a larger pilot jet such as #45 and it must be genuine Mikuni.

This is a very common problem with Yamaha motorcycles of this vintage. It’s low voltage to the coils with a cold engine and the starter engaged. I have a 1983 XS650 with the same issue. In fact, it won’t cold start at all with the electric leg. Fortunately, it is fitted with a kickstarter and usually starts on the first or second kick if I hold my mouth right. Do you have the kick lever for this bike? You can use it to prove my theory. My XS650SK starts just fine with the electric leg after it’s warmed up. I also rebuilt my starter. No change.
A friend of mine fixed the issue with a relay. It’s grounded through the kill switch. It puts battery voltage directly to the coils. This is an XS750 with the exact symptoms described here.

[Radioguylogs]

Hmmmm....my XS750 has always been hard to start unless the battery is in perfect shape.......

[Ccitis]

This is a very common problem with Yamaha motorcycles of this vintage. It’s low voltage to the coils with a cold engine and the starter engaged. I have a 1983 XS650 with the same issue. In fact, it won’t cold start at all with the electric leg. Fortunately, it is fitted with a kickstarter and usually starts on the first or second kick if I hold my mouth right. Do you have the kick lever for this bike? You can use it to prove my theory. My XS650SK starts just fine with the electric leg after it’s warmed up. I also rebuilt my starter. No change.
A friend of mine fixed the issue with a relay. It’s grounded through the kill switch. It puts battery voltage directly to the coils. This is an XS750 with the exact symptoms described here.

There was a guy at a parts swap selling little relays to put volts direct to the coils. Look up "XS1100 Coil Relay mod". Could help you out. My bike is running well, and I am nervous to cut into the wiring harness... so have not installed yet. Had to buy it though. 20 bucks to support a dude who is trying to support XS bikes seems like money well spent.

[bikerphil]

Here is a simple mod for full power coil voltage using a standard 4 pin Bosch relay...