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View Full Version : Spark plugs and 30+ y/o inline 4s



WarMullet
05-25-2016, 6:07 PM
So the information about copper, iridium and platinum core plugs in relation to old bikes has been a bear to find. I read ALOT of posts about people swearing by expensive plugs and then I see old timers saying older bikes run better on OEM copper.

So I suppose this question is two fold, my build is a CB900C that I have a Dyna coil ignition and 2.2 ohm coils on. On top of that I am not seeing anything saying I should use resistor type plugs or not. Will it benefit me to use one type over the other. And the other side to that, which should be used in a stock application?

Hope this hasn't been covered ad nauseam and I just didn't find it.

Update:

I have not seen anything about riching the air/fuel mixture or other carb tweaks to run better on the 10% ethanol fuels. Per haps this is not even a concern, but since I don't see anyone asking, I will.

bellam87
05-25-2016, 8:05 PM
I've been running ngk d8ea's in my cb750. Dunno if they work with the 900's but ngk is the only way my bike will have it when it comes to plugs. I did the gm coil swap and it burned up all of the cheaper brands (auto lite, champion, etc...)

WarMullet
05-25-2016, 8:12 PM
I totally appreciate the insight. I am seeing that NGK is really the only brand that contends for our bikes.

TesticularCancer
05-25-2016, 11:06 PM
as with cars, i find that the platinum and iridium plugs only work with ignition systems that were designed for them. i found that the spark seems weaker in an old bike with the new style plugs.

Sky
05-26-2016, 6:19 AM
Generally,
Use the recommended plugs/gap for whatever ignition your running. AKA read the instructions.
Modified engines might need a different heat range and/or gap than stockers.
Most electronic ignitions require resistor type plugs or caps to quell radio interference.
Fuel mix, octane and quality can be a variable in what works in your bike.

Specifically,
If your hotrod likes it a little rich on the bottom end.The iridiums might be worth a shot due to their "self cleaning", helps keep 'em from fouling.
If you've got a high comp setup, it's worth spending some time with a file to round those sharp edges off the ground strap.
Indexing plugs isn't going to hurt.
Don't forget those spark plug wires!

OleDirtyDoc
05-26-2016, 12:13 PM
I've been running ngk d8ea's in my cb750. Dunno if they work with the 900's but ngk is the only way my bike will have it when it comes to plugs. I did the gm coil swap and it burned up all of the cheaper brands (auto lite, champion, etc...)

X2 on my cb750

ContractKiller
05-26-2016, 3:15 PM
I run NGK D8EAs in my Cb900 and I haven't had any problems with them.

WarMullet
05-26-2016, 3:33 PM
I run NGK D8EAs in my Cb900 and I haven't had any problems with them. My man!

Good to know brother. Yeah I need to re-re-read those Dyna instructions because I don't remember seeing anything specific.

ContractKiller
05-26-2016, 5:11 PM
Yeah I don't know much about the Dyna ignition, mine is still factory. Probably will stay that way unless I need to replace anything, but then again we'll see how she runs after I get the new carb setup done.

CarloFreeze
05-26-2016, 8:51 PM
It's unlikely that you need a resistor plug. It's more likely that you need resistor caps. The Honda's and Suzuki's typically run 5k ohm spark plug caps and whatever specified NGK plug heat range the bike calls for. I was running NGK D8EA's in my 1978 CB750K, but then I switched to Denso X-24EU plugs as they have less of a tendency to foul and I've never looked back.

As far as resistor caps go, the resistance is there to lengthen that the spark duration so that there is a more complete burn in the cylinder. It the way these engines were designed. There's also suppression core plug wires, I haven't heard of any situation that requires them at all on the old inline 4's.

WarMullet
05-26-2016, 8:55 PM
It's unlikely that you need a resistor plug. It's more likely that you need resistor caps. The Honda's and Suzuki's typically run 5k ohm spark plug caps and whatever specified NGK plug heat range the bike calls for. I was running NGK D8EA's in my 1978 CB750K, but then I switched to Denso X-24EU plugs as they have less of a tendency to foul and I've never looked back.

As far as resistor caps go, the resistance is there to lengthen that the spark duration so that there is a more complete burn in the cylinder. It the way these engines were designed. There's also suppression core plug wires, I haven't heard of any situation that requires them at all on the old inline 4's.

I appreciate the wisdom mate. Have anything for tuning carbs and whatnot for ethanol in fuel?

CarloFreeze
05-26-2016, 11:24 PM
The ethanol question only really matters so much because it's everywhere anyways. You can always run higher octane if it tickles your pickle. More important than fuel is making sure you've done a really good job cleaning the carburetors and synchronizing them because that's really what makes these bikes run their best. Making sure the carb manifolds boots are in good shape is important. Making sure you have good spark plug caps is important, unless they're new already.

In terms of tuning I really can't offer advice without knowing what your setup is.

ContractKiller
05-27-2016, 9:48 AM
The engine won't care if you feed it ethanol but these engines are tuned to run lean on regular gas as is so you might need to fatten up the fuel mix a little. You'll also not want to let the ethanol sit in the bike for too long since not only will it attract water but the alcohol will evaporate and leave behind a residue that will gum up your carbs. Adding some Stabil or even some Seafoam will help with that. Also keep in mind with constant use ethanol can eventually damage the rubber seals and such in your carbs. Your best bet is to just avoid running ethanol if at all possible but if you have to run it be mindful of these things.

WarMullet
05-27-2016, 3:22 PM
I thought about using higher octane, but isn't there still the propensity of the fuel being old due to inflated fuel prices? I'm always worried mid and premium octanes have been sitting considerably longer than the stuff everyone and their grandma uses daily. Don't want crap fuel in my bike regardless.

CarloFreeze
05-27-2016, 7:44 PM
I thought about using higher octane, but isn't there still the propensity of the fuel being old due to inflated fuel prices? I'm always worried mid and premium octanes have been sitting considerably longer than the stuff everyone and their grandma uses daily. Don't want crap fuel in my bike regardless.

I'm on the fence on this one. Sometimes I run higher octane and sometimes I don't. My CB750 is my daily driver so I'm never letting it sit. If I'm going on a long trip I'll usually just throw in 87 since it's going to get used up anyways. Also that's the gas that most people buy so it's the "freshest" if you can call it that hahahaha. The effects of ethanol aren't big enough to really throw into a tuning debate in my opinion unless you really had a solid source to get ethanol free gas all the time which I'm guessing you don't.

WarMullet
05-27-2016, 8:41 PM
I'm on the fence on this one. Sometimes I run higher octane and sometimes I don't. My CB750 is my daily driver so I'm never letting it sit. If I'm going on a long trip I'll usually just throw in 87 since it's going to get used up anyways. Also that's the gas that most people buy so it's the "freshest" if you can call it that hahahaha. The effects of ethanol aren't big enough to really throw into a tuning debate in my opinion unless you really had a solid source to get ethanol free gas all the time which I'm guessing you don't. You are correct... I'm in an ass rape dont fart for fear of the EPA emissions zone. I suppose just riching up the mixture is the best bet for a happy bike since they were tuned so damn lean from the factory.

ContractKiller
05-28-2016, 5:16 PM
I would recommend not running ethanol if you can get away with it. If you have to run it, provided the carbs still have a factory tune, then yeah, you'll probably want to fatten the mix a little. You could get lucky, though. It's entirely possible the carb settings have been changed and it'll run just fine. I know my bike tolerated it just fine when I stopped at a gad station akd all they had was ethanol, but that was the only time I ran it. Best way to find out if you neednto adjust the mix is drain your tank and throw a gallon of ethanol in it, and take her for a spin. If you don't hear any knocking or pinging you're probably fine. If you happen to have fresh spark plugs in there you can check them, too. Like I said before though, these carbs are not made to run ethanol and it can eventually ruin the rubber components. Just keep that in mind.